diff --git a/sledge/cli/domain_itv.ml b/sledge/cli/domain_itv.ml
index 6fd0cbe24..fd5037c02 100644
--- a/sledge/cli/domain_itv.ml
+++ b/sledge/cli/domain_itv.ml
@@ -31,7 +31,7 @@ let bindings (itv : t) =
 
 let sexp_of_t (itv : t) =
   let sexps =
-    Array.fold (bindings itv) ~init:[] ~f:(fun acc (v, {inf; sup}) ->
+    Array.fold_right (bindings itv) [] ~f:(fun (v, {inf; sup}) acc ->
         Sexp.List
           [ Sexp.Atom (Var.to_string v)
           ; Sexp.Atom (Scalar.to_string inf)
@@ -166,40 +166,39 @@ let exec_assume q e =
   | _ -> Some q
 
 (** existentially quantify killed register [r] out of state [q] *)
-let exec_kill q r =
+let exec_kill r q =
   let apron_v = apron_var_of_reg r in
   if Environment.mem_var (Abstract1.env q) apron_v then
     Abstract1.forget_array (Lazy.force man) q [|apron_v|] false
   else q
 
 (** perform a series [move_vec] of reg:=exp moves at state [q] *)
-let exec_move q move_vec =
+let exec_move move_vec q =
   let defs, uses =
-    IArray.fold move_vec ~init:(Llair.Reg.Set.empty, Llair.Reg.Set.empty)
-      ~f:(fun (defs, uses) (r, e) ->
+    IArray.fold move_vec (Llair.Reg.Set.empty, Llair.Reg.Set.empty)
+      ~f:(fun (r, e) (defs, uses) ->
         ( Llair.Reg.Set.add r defs
-        , Llair.Exp.fold_regs e ~init:uses ~f:(Fun.flip Llair.Reg.Set.add)
-        ) )
+        , Llair.Exp.fold_regs ~f:Llair.Reg.Set.add e uses ) )
   in
   assert (Llair.Reg.Set.disjoint defs uses) ;
-  IArray.fold move_vec ~init:q ~f:(fun a (r, e) -> assign r e a)
+  IArray.fold ~f:(fun (r, e) q -> assign r e q) move_vec q
 
-let exec_inst q i =
+let exec_inst i q =
   match (i : Llair.inst) with
-  | Move {reg_exps; loc= _} -> Some (exec_move q reg_exps)
+  | Move {reg_exps; loc= _} -> Some (exec_move reg_exps q)
   | Store {ptr; exp; len= _; loc= _} -> (
     match Llair.Reg.of_exp ptr with
     | Some reg -> Some (assign reg exp q)
     | None -> Some q )
   | Load {reg; ptr; len= _; loc= _} -> Some (assign reg ptr q)
-  | Nondet {reg= Some reg; msg= _; loc= _} -> Some (exec_kill q reg)
+  | Nondet {reg= Some reg; msg= _; loc= _} -> Some (exec_kill reg q)
   | Nondet {reg= None; msg= _; loc= _}
    |Alloc _ | Memset _ | Memcpy _ | Memmov _ | Free _ ->
       Some q
   | Abort _ -> None
 
 (** Treat any intrinsic function as havoc on the return register [aret] *)
-let exec_intrinsic ~skip_throw:_ pre aret i _ =
+let exec_intrinsic ~skip_throw:_ aret i _ pre =
   let name = Llair.Reg.name i in
   if
     List.exists
@@ -224,7 +223,9 @@ let exec_intrinsic ~skip_throw:_ pre aret i _ =
       ; "__cxa_allocate_exception"
       ; "_ZN5folly13usingJEMallocEv" ]
       ~f:(String.equal name)
-  then Option.map ~f:(Option.return << exec_kill pre) aret
+  then
+    let+ aret = aret in
+    Some (exec_kill aret pre)
   else None
 
 type from_call = {areturn: Llair.Reg.t option; caller_q: t}
@@ -235,7 +236,7 @@ let recursion_beyond_bound = `prune
 (** existentially quantify locals *)
 let post locals _ (q : t) =
   let locals =
-    Llair.Reg.Set.fold locals ~init:[] ~f:(fun r a ->
+    Llair.Reg.Set.fold locals [] ~f:(fun r a ->
         let v = apron_var_of_reg r in
         if Environment.mem_var q.env v then v :: a else a )
     |> Array.of_list
@@ -264,7 +265,7 @@ let retn _ freturn {areturn; caller_q} callee_q =
       Abstract1.rename_array man result
         [|apron_var_of_reg fret|]
         [|apron_var_of_reg aret|]
-  | Some aret, None -> exec_kill caller_q aret
+  | Some aret, None -> exec_kill aret caller_q
   | None, _ -> caller_q
 
 (** map actuals to formals (via temporary registers), stash constraints on
@@ -280,11 +281,9 @@ let call ~summaries ~globals:_ ~actuals ~areturn ~formals ~freturn:_
       Llair.Reg.program (Llair.Reg.typ r) ("__tmp__" ^ Llair.Reg.name r)
     in
     let args = List.combine_exn formals actuals in
-    let q' =
-      List.fold args ~init:q ~f:(fun q (f, a) -> assign (mangle f) a q)
-    in
+    let q' = List.fold ~f:(fun (f, a) q -> assign (mangle f) a q) args q in
     let callee_env =
-      List.fold formals ~init:([], []) ~f:(fun (is, fs) f ->
+      List.fold formals ([], []) ~f:(fun f (is, fs) ->
           match apron_typ_of_llair_typ (Llair.Reg.typ f) with
           | None -> (is, fs)
           | Some Texpr1.Int -> (apron_var_of_reg (mangle f) :: is, fs)
diff --git a/sledge/cli/frontend.ml b/sledge/cli/frontend.ml
index c62988753..aab2ee2c6 100644
--- a/sledge/cli/frontend.ml
+++ b/sledge/cli/frontend.ml
@@ -865,10 +865,10 @@ let xlate_jump :
   match xlate_jump_ reg_exps (Llvm.instr_begin dst) with
   | [] -> ([], jmp, blocks)
   | rev_reg_pre_exps ->
-      let rev_pre, rev_reg_exps =
-        List.fold_map rev_reg_pre_exps ~init:[]
-          ~f:(fun rev_pre (reg, (pre, exp)) ->
-            (List.rev_append pre rev_pre, (reg, exp)) )
+      let rev_reg_exps, rev_pre =
+        List.fold_map rev_reg_pre_exps []
+          ~f:(fun (reg, (pre, exp)) rev_pre ->
+            ((reg, exp), List.rev_append pre rev_pre) )
       in
       let mov =
         Inst.move ~reg_exps:(IArray.of_list_rev rev_reg_exps) ~loc
diff --git a/sledge/cli/sledge_cli.ml b/sledge/cli/sledge_cli.ml
index 34ac51cdb..725e76717 100644
--- a/sledge/cli/sledge_cli.ml
+++ b/sledge/cli/sledge_cli.ml
@@ -95,8 +95,8 @@ let used_globals pgm preanalyze : Domain_used_globals.r =
       (Llair.Reg.Map.map summary_table ~f:Llair.Reg.Set.union_list)
   else
     Declared
-      (IArray.fold pgm.globals ~init:Llair.Reg.Set.empty ~f:(fun acc g ->
-           Llair.Reg.Set.add g.reg acc ))
+      (IArray.fold pgm.globals Llair.Reg.Set.empty ~f:(fun g ->
+           Llair.Reg.Set.add g.reg ))
 
 let analyze =
   let%map_open bound =
diff --git a/sledge/cli/smtlib.ml b/sledge/cli/smtlib.ml
index df9320019..e5f8b04c0 100644
--- a/sledge/cli/smtlib.ml
+++ b/sledge/cli/smtlib.ml
@@ -45,9 +45,9 @@ let assert_term term =
   let top = top () in
   top.asserts <- term :: top.asserts
 
-let rec x_let init nes =
-  List.fold nes ~init ~f:(fun n (name, term) ->
-      VarEnv.add_exn ~key:name ~data:(x_trm init term) n )
+let rec x_let env nes =
+  List.fold nes env ~f:(fun (name, term) ->
+      VarEnv.add_exn ~key:name ~data:(x_trm env term) )
 
 and x_trm : var_env -> Smt.Ast.term -> Term.t =
  fun n term ->
@@ -60,13 +60,13 @@ and x_trm : var_env -> Smt.Ast.term -> Term.t =
         try Term.rational (Q.of_float (Float.of_string_exn s))
         with _ -> fail "not a rational: %a" Smt.Ast.pp_term term () ) ) )
   | Arith (Add, e :: es) ->
-      List.fold ~f:(fun s e -> Term.add s (x_trm n e)) ~init:(x_trm n e) es
+      List.fold ~f:(fun e -> Term.add (x_trm n e)) es (x_trm n e)
   | Arith (Minus, e :: es) ->
-      List.fold ~f:(fun s e -> Term.sub s (x_trm n e)) ~init:(x_trm n e) es
+      List.fold ~f:(fun e -> Term.sub (x_trm n e)) es (x_trm n e)
   | Arith (Mult, es) -> (
     match List.map ~f:(x_trm n) es with
     | e :: es ->
-        List.fold es ~init:e ~f:(fun p e ->
+        List.fold es e ~f:(fun e p ->
             match Term.get_const e with
             | Some q -> Term.mulq q p
             | None -> (
@@ -77,7 +77,7 @@ and x_trm : var_env -> Smt.Ast.term -> Term.t =
   | Arith (Div, es) -> (
     match List.map ~f:(x_trm n) es with
     | e :: es ->
-        List.fold es ~init:e ~f:(fun p e ->
+        List.fold es e ~f:(fun e p ->
             match Term.get_const e with
             | Some q -> Term.mulq (Q.inv q) p
             | None -> fail "nonlinear: %a" Smt.Ast.pp_term term () )
diff --git a/sledge/cli/version.ml b/sledge/cli/version.ml
index 71f1b5577..19b1998ad 100644
--- a/sledge/cli/version.ml
+++ b/sledge/cli/version.ml
@@ -34,7 +34,7 @@ let build_info =
     |> List.sort ~cmp:[%compare: string * string]
   in
   let max_length =
-    List.fold_left libs ~init:0 ~f:(fun n (name, _) ->
+    List.fold_left libs 0 ~f:(fun n (name, _) ->
         max n (String.length name) )
   in
   String.concat ~sep:"\n"
diff --git a/sledge/nonstdlib/NS.mli b/sledge/nonstdlib/NS.mli
index 4228701ab..fecc6e055 100644
--- a/sledge/nonstdlib/NS.mli
+++ b/sledge/nonstdlib/NS.mli
@@ -137,6 +137,7 @@ module Either : sig
   val right : 'a -> ('b, 'a) t
 end
 
+module Pair = Containers.Pair
 module List = List
 module Array = Array
 module IArray = IArray
diff --git a/sledge/nonstdlib/NS0.ml b/sledge/nonstdlib/NS0.ml
index 3060325df..d206531a9 100644
--- a/sledge/nonstdlib/NS0.ml
+++ b/sledge/nonstdlib/NS0.ml
@@ -127,6 +127,7 @@ module Either = struct
   let right v = Right v
 end
 
+module Pair = Containers.Pair
 module FHeap = Fheap [@@warning "-49"]
 module HashQueue = Core_kernel.Hash_queue
 
diff --git a/sledge/nonstdlib/array.ml b/sledge/nonstdlib/array.ml
index 498d92fba..4966a71a7 100644
--- a/sledge/nonstdlib/array.ml
+++ b/sledge/nonstdlib/array.ml
@@ -74,29 +74,31 @@ let iter xs ~f = iter ~f xs
 let iteri xs ~f = iteri ~f xs
 let exists xs ~f = exists ~f xs
 let for_all xs ~f = for_all ~f xs
-let fold xs ~init ~f = fold ~f ~init xs
-let fold_right xs ~init ~f = fold_right ~f ~init xs
-let fold_map xs ~init ~f = fold_map ~f ~init xs
+let fold xs init ~f = fold ~f:(fun s x -> f x s) ~init xs
+let fold_right xs init ~f = fold_right ~f ~init xs
 
-let fold_map_until xs ~init ~f ~finish =
+let fold_map xs init ~f =
+  Pair.swap (fold_map ~f:(fun s x -> Pair.swap (f x s)) ~init xs)
+
+let fold_map_until xs s ~f ~finish =
   let l = length xs in
-  if l = 0 then finish (init, [||])
+  if l = 0 then finish ([||], s)
   else
-    match f init xs.(0) with
+    match f xs.(0) s with
     | `Stop r -> r
-    | `Continue (s, y) ->
+    | `Continue (y, s) ->
         let ys = make l y in
         let rec fold_map_until_ s i =
-          if i = l then finish (s, ys)
+          if i = l then finish (ys, s)
           else
-            match f s xs.(i) with
+            match f xs.(i) s with
             | `Stop r -> r
-            | `Continue (s, y) ->
+            | `Continue (y, s) ->
                 ys.(i) <- y ;
                 fold_map_until_ s (i + 1)
         in
         fold_map_until_ s 1
 
 let for_all2_exn xs ys ~f = for_all2 ~f xs ys
-let to_list_rev_map xs ~f = fold ~f:(fun ys x -> f x :: ys) ~init:[] xs
+let to_list_rev_map xs ~f = fold ~f:(fun x ys -> f x :: ys) xs []
 let pp sep pp_elt fs a = List.pp sep pp_elt fs (to_list a)
diff --git a/sledge/nonstdlib/array.mli b/sledge/nonstdlib/array.mli
index 66d4effba..f449ddf1d 100644
--- a/sledge/nonstdlib/array.mli
+++ b/sledge/nonstdlib/array.mli
@@ -29,15 +29,15 @@ val iteri : 'a t -> f:(int -> 'a -> unit) -> unit
 val exists : 'a t -> f:('a -> bool) -> bool
 val for_all : 'a t -> f:('a -> bool) -> bool
 val for_all2_exn : 'a t -> 'b t -> f:('a -> 'b -> bool) -> bool
-val fold : 'a array -> init:'s -> f:('s -> 'a -> 's) -> 's
-val fold_right : 'a t -> init:'s -> f:('a -> 's -> 's) -> 's
-val fold_map : 'a t -> init:'s -> f:('s -> 'a -> 's * 'b) -> 's * 'b t
+val fold : 'a t -> 's -> f:('a -> 's -> 's) -> 's
+val fold_right : 'a t -> 's -> f:('a -> 's -> 's) -> 's
+val fold_map : 'a t -> 's -> f:('a -> 's -> 'b * 's) -> 'b t * 's
 
 val fold_map_until :
      'a t
-  -> init:'s
-  -> f:('s -> 'a -> [`Continue of 's * 'b | `Stop of 'c])
-  -> finish:('s * 'b t -> 'c)
+  -> 's
+  -> f:('a -> 's -> [`Continue of 'b * 's | `Stop of 'c])
+  -> finish:('b t * 's -> 'c)
   -> 'c
 
 val to_list_rev_map : 'a array -> f:('a -> 'b) -> 'b list
diff --git a/sledge/nonstdlib/hashTable.ml b/sledge/nonstdlib/hashTable.ml
index 2f838b702..67fb5cebc 100644
--- a/sledge/nonstdlib/hashTable.ml
+++ b/sledge/nonstdlib/hashTable.ml
@@ -37,7 +37,5 @@ module Make (Key : HashedType) = struct
     Option.get_exn !found
 
   let iteri tbl ~f = iter (fun key data -> f ~key ~data) tbl
-
-  let fold tbl ~init ~f =
-    fold (fun key data acc -> f ~key ~data acc) tbl init
+  let fold tbl s ~f = fold (fun key data acc -> f ~key ~data acc) tbl s
 end
diff --git a/sledge/nonstdlib/hashTable_intf.ml b/sledge/nonstdlib/hashTable_intf.ml
index ac8554cc0..21767961b 100644
--- a/sledge/nonstdlib/hashTable_intf.ml
+++ b/sledge/nonstdlib/hashTable_intf.ml
@@ -19,5 +19,5 @@ module type S = sig
   val find : 'a t -> key -> 'a option
   val find_or_add : 'a t -> key -> default:(unit -> 'a) -> 'a
   val iteri : 'a t -> f:(key:key -> data:'a -> unit) -> unit
-  val fold : 'a t -> init:'s -> f:(key:key -> data:'a -> 's -> 's) -> 's
+  val fold : 'a t -> 's -> f:(key:key -> data:'a -> 's -> 's) -> 's
 end
diff --git a/sledge/nonstdlib/iArray.mli b/sledge/nonstdlib/iArray.mli
index 756258ae6..94ad7576c 100644
--- a/sledge/nonstdlib/iArray.mli
+++ b/sledge/nonstdlib/iArray.mli
@@ -57,15 +57,13 @@ val iteri : 'a t -> f:(int -> 'a -> unit) -> unit
 val exists : 'a t -> f:('a -> bool) -> bool
 val for_all : 'a t -> f:('a -> bool) -> bool
 val for_all2_exn : 'a t -> 'b t -> f:('a -> 'b -> bool) -> bool
-val fold : 'a t -> init:'s -> f:('s -> 'a -> 's) -> 's
-val fold_right : 'a t -> init:'s -> f:('a -> 's -> 's) -> 's
-
-val fold_map :
-  'a t -> init:'accum -> f:('accum -> 'a -> 'accum * 'b) -> 'accum * 'b t
+val fold : 'a t -> 's -> f:('a -> 's -> 's) -> 's
+val fold_right : 'a t -> 's -> f:('a -> 's -> 's) -> 's
+val fold_map : 'a t -> 's -> f:('a -> 's -> 'b * 's) -> 'b t * 's
 
 val fold_map_until :
      'a t
-  -> init:'s
-  -> f:('s -> 'a -> [`Continue of 's * 'b | `Stop of 'c])
-  -> finish:('s * 'b t -> 'c)
+  -> 's
+  -> f:('a -> 's -> [`Continue of 'b * 's | `Stop of 'c])
+  -> finish:('b t * 's -> 'c)
   -> 'c
diff --git a/sledge/nonstdlib/iter.ml b/sledge/nonstdlib/iter.ml
index bc9b7cc32..66be7f802 100644
--- a/sledge/nonstdlib/iter.ml
+++ b/sledge/nonstdlib/iter.ml
@@ -37,6 +37,7 @@ let pop seq =
 let find_map seq ~f = find_map ~f seq
 let find seq ~f = find (CCOpt.if_ f) seq
 let find_exn seq ~f = CCOpt.get_exn (find ~f seq)
+let fold seq init ~f = fold ~f:(fun s x -> f x s) ~init seq
 
 let contains_dup (type elt) seq ~cmp =
   let module S = CCSet.Make (struct
@@ -46,41 +47,41 @@ let contains_dup (type elt) seq ~cmp =
   end) in
   let exception Found_dup in
   try
-    fold ~init:S.empty seq ~f:(fun elts x ->
+    fold seq S.empty ~f:(fun x elts ->
         let elts' = S.add x elts in
         if elts' == elts then raise_notrace Found_dup else elts' )
     |> ignore ;
     false
   with Found_dup -> true
 
-let fold_opt seq ~init ~f =
-  let state = ref init in
+let fold_opt seq s ~f =
+  let state = ref s in
   let exception Stop in
   try
     seq (fun x ->
-        match f !state x with
+        match f x !state with
         | Some s -> state := s
         | None -> raise_notrace Stop ) ;
     Some !state
   with Stop -> None
 
-let fold_until (type res) seq ~init ~f ~finish =
-  let state = ref init in
+let fold_until (type res) seq s ~f ~finish =
+  let state = ref s in
   let exception Stop of res in
   try
     seq (fun x ->
-        match f !state x with
+        match f x !state with
         | `Continue s -> state := s
         | `Stop r -> raise_notrace (Stop r) ) ;
     finish !state
   with Stop r -> r
 
-let fold_result (type s e) seq ~init ~f =
-  let state = ref init in
+let fold_result (type s e) seq s ~f =
+  let state = ref s in
   let exception Stop of (s, e) result in
   try
     seq (fun x ->
-        match f !state x with
+        match f x !state with
         | Ok s -> state := s
         | Error _ as e -> raise_notrace (Stop e) ) ;
     Ok !state
diff --git a/sledge/nonstdlib/iter.mli b/sledge/nonstdlib/iter.mli
index 0968d6fb1..d52c3645b 100644
--- a/sledge/nonstdlib/iter.mli
+++ b/sledge/nonstdlib/iter.mli
@@ -66,18 +66,19 @@ val find : 'a t -> f:('a -> bool) -> 'a option
 val find_exn : 'a t -> f:('a -> bool) -> 'a
 val find_map : 'a iter -> f:('a -> 'b option) -> 'b option
 val contains_dup : 'a iter -> cmp:('a -> 'a -> int) -> bool
+val fold : 'a t -> 's -> f:('a -> 's -> 's) -> 's
 
-val fold_opt : 'a t -> init:'s -> f:('s -> 'a -> 's option) -> 's option
-(** [fold_option t ~init ~f] is a short-circuiting version of [fold] that
-    runs in the [Option] monad. If [f] returns [None], [None] is returned
-    without any additional invocations of [f]. *)
+val fold_opt : 'a t -> 's -> f:('a -> 's -> 's option) -> 's option
+(** [fold_opt t s ~f] is a short-circuiting version of [fold] that runs in
+    the [Option] monad. If [f] returns [None], [None] is returned without
+    any additional invocations of [f]. *)
 
 val fold_until :
      'a t
-  -> init:'s
-  -> f:('s -> 'a -> [`Continue of 's | `Stop of 'b])
+  -> 's
+  -> f:('a -> 's -> [`Continue of 's | `Stop of 'b])
   -> finish:('s -> 'b)
   -> 'b
 
 val fold_result :
-  'a t -> init:'s -> f:('s -> 'a -> ('s, 'e) result) -> ('s, 'e) result
+  'a t -> 's -> f:('a -> 's -> ('s, 'e) result) -> ('s, 'e) result
diff --git a/sledge/nonstdlib/list.ml b/sledge/nonstdlib/list.ml
index e6cba4977..107c229cb 100644
--- a/sledge/nonstdlib/list.ml
+++ b/sledge/nonstdlib/list.ml
@@ -51,12 +51,19 @@ let group_by seq ~hash ~eq = group_by ~hash ~eq seq
 let join_by ~eq ~hash k1 k2 ~merge = join_by ~eq ~hash k1 k2 ~merge
 let join_all_by ~eq ~hash k1 k2 ~merge = join_all_by ~eq ~hash k1 k2 ~merge
 let group_join_by ~eq ~hash = group_join_by ~eq ~hash
-let fold xs ~init ~f = fold_left ~f ~init xs
+let fold xs init ~f = fold_left ~f:(fun s x -> f x s) ~init xs
+let fold_left xs init ~f = fold_left ~f ~init xs
+let fold_right xs init ~f = fold_right ~f ~init xs
 
 let reduce xs ~f =
-  match xs with [] -> None | x :: xs -> Some (fold xs ~init:x ~f)
+  match xs with [] -> None | x :: xs -> Some (fold ~f xs x)
+
+let fold_map xs init ~f =
+  Pair.swap (fold_map ~f:(fun s x -> Pair.swap (f x s)) ~init xs)
+
+let fold2_exn xs ys init ~f =
+  fold_left2 ~f:(fun s x y -> f x y s) ~init xs ys
 
-let fold2_exn xs ys ~init ~f = fold_left2 ~f ~init xs ys
 let group_succ ~eq xs = group_succ ~eq:(fun y x -> eq x y) xs
 
 let symmetric_diff ~cmp xs ys =
diff --git a/sledge/nonstdlib/list.mli b/sledge/nonstdlib/list.mli
index 693585bb8..03a6f3a94 100644
--- a/sledge/nonstdlib/list.mli
+++ b/sledge/nonstdlib/list.mli
@@ -86,9 +86,12 @@ val group_join_by :
   -> 'b t
   -> ('a * 'b list) t
 
-val fold : 'a list -> init:'s -> f:('s -> 'a -> 's) -> 's
+val fold : 'a list -> 's -> f:('a -> 's -> 's) -> 's
+val fold_left : 'a list -> 's -> f:('s -> 'a -> 's) -> 's
+val fold_right : 'a list -> 's -> f:('a -> 's -> 's) -> 's
 val reduce : 'a t -> f:('a -> 'a -> 'a) -> 'a option
-val fold2_exn : 'a t -> 'b t -> init:'s -> f:('s -> 'a -> 'b -> 's) -> 's
+val fold_map : 'a t -> 's -> f:('a -> 's -> 'b * 's) -> 'b t * 's
+val fold2_exn : 'a t -> 'b t -> 's -> f:('a -> 'b -> 's -> 's) -> 's
 
 val symmetric_diff :
   cmp:('a -> 'a -> int) -> 'a t -> 'a t -> ('a, 'a) Either.t t
diff --git a/sledge/nonstdlib/map.ml b/sledge/nonstdlib/map.ml
index 83346078b..9c4a1807a 100644
--- a/sledge/nonstdlib/map.ml
+++ b/sledge/nonstdlib/map.ml
@@ -158,7 +158,7 @@ end) : S with type key = Key.t = struct
   let iteri m ~f = M.iter (fun key data -> f ~key ~data) m
   let existsi m ~f = M.exists (fun key data -> f ~key ~data) m
   let for_alli m ~f = M.for_all (fun key data -> f ~key ~data) m
-  let fold m ~init ~f = M.fold (fun key data acc -> f ~key ~data acc) m init
+  let fold m s ~f = M.fold (fun key data acc -> f ~key ~data acc) m s
   let keys = M.keys
   let values = M.values
   let to_iter = M.to_iter
diff --git a/sledge/nonstdlib/map_intf.ml b/sledge/nonstdlib/map_intf.ml
index 035324103..939096e20 100644
--- a/sledge/nonstdlib/map_intf.ml
+++ b/sledge/nonstdlib/map_intf.ml
@@ -105,7 +105,7 @@ module type S = sig
   val iteri : 'a t -> f:(key:key -> data:'a -> unit) -> unit
   val existsi : 'a t -> f:(key:key -> data:'a -> bool) -> bool
   val for_alli : 'a t -> f:(key:key -> data:'a -> bool) -> bool
-  val fold : 'a t -> init:'s -> f:(key:key -> data:'a -> 's -> 's) -> 's
+  val fold : 'a t -> 's -> f:(key:key -> data:'a -> 's -> 's) -> 's
 
   (** {1 Convert} *)
 
diff --git a/sledge/nonstdlib/multiset.ml b/sledge/nonstdlib/multiset.ml
index 7f34a9085..aae21459a 100644
--- a/sledge/nonstdlib/multiset.ml
+++ b/sledge/nonstdlib/multiset.ml
@@ -26,9 +26,8 @@ struct
 
   let hash_fold_t hash_fold_elt s m =
     let hash_fold_mul s i = Hash.fold_int s (Mul.hash i) in
-    M.fold m
-      ~init:(Hash.fold_int s (M.length m))
-      ~f:(fun ~key ~data state ->
+    let init = Hash.fold_int s (M.length m) in
+    M.fold m init ~f:(fun ~key ~data state ->
         hash_fold_mul (hash_fold_elt state key) data )
 
   let sexp_of_t s =
@@ -39,10 +38,10 @@ struct
   let t_of_sexp elt_of_sexp sexp =
     List.fold_left
       ~f:(fun m (key, data) -> M.add_exn ~key ~data m)
-      ~init:M.empty
       (List.t_of_sexp
          (Sexplib.Conv.pair_of_sexp elt_of_sexp Mul.t_of_sexp)
          sexp)
+      M.empty
 
   let pp sep pp_elt fs s =
     List.pp sep pp_elt fs (Iter.to_list (M.to_iter s))
@@ -71,7 +70,7 @@ struct
   let map m ~f =
     let m' = empty in
     let m, m' =
-      M.fold m ~init:(m, m') ~f:(fun ~key:x ~data:i (m, m') ->
+      M.fold m (m, m') ~f:(fun ~key:x ~data:i (m, m') ->
           let x', i' = f x i in
           if x' == x then
             if Mul.equal i' i then (m, m') else (M.add ~key:x ~data:i' m, m')
@@ -85,7 +84,7 @@ struct
   let flat_map m ~f =
     let m' = empty in
     let m, m' =
-      M.fold m ~init:(m, m') ~f:(fun ~key:x ~data:i (m, m') ->
+      M.fold m (m, m') ~f:(fun ~key:x ~data:i (m, m') ->
           let d = f x i in
           match M.only_binding d with
           | Some (x', i') ->
@@ -112,5 +111,5 @@ struct
   let iter m ~f = M.iteri ~f:(fun ~key ~data -> f key data) m
   let exists m ~f = M.existsi ~f:(fun ~key ~data -> f key data) m
   let for_all m ~f = M.for_alli ~f:(fun ~key ~data -> f key data) m
-  let fold m ~init ~f = M.fold ~f:(fun ~key ~data -> f key data) m ~init
+  let fold m s ~f = M.fold ~f:(fun ~key ~data -> f key data) m s
 end
diff --git a/sledge/nonstdlib/multiset_intf.ml b/sledge/nonstdlib/multiset_intf.ml
index fd02a86d9..2ae34de6c 100644
--- a/sledge/nonstdlib/multiset_intf.ml
+++ b/sledge/nonstdlib/multiset_intf.ml
@@ -115,6 +115,6 @@ module type S = sig
   val for_all : t -> f:(elt -> mul -> bool) -> bool
   (** Test whether all elements satisfy a predicate. *)
 
-  val fold : t -> init:'s -> f:(elt -> mul -> 's -> 's) -> 's
+  val fold : t -> 's -> f:(elt -> mul -> 's -> 's) -> 's
   (** Fold over the elements in ascending order. *)
 end
diff --git a/sledge/nonstdlib/option.ml b/sledge/nonstdlib/option.ml
index a319c519d..5588f7930 100644
--- a/sledge/nonstdlib/option.ml
+++ b/sledge/nonstdlib/option.ml
@@ -20,4 +20,4 @@ let bind xo ~f = bind xo f
 let iter xo ~f = iter f xo
 let exists xo ~f = exists f xo
 let for_all xo ~f = for_all f xo
-let fold xo ~init ~f = fold f init xo
+let fold xo s ~f = fold (fun x s -> f s x) s xo
diff --git a/sledge/nonstdlib/option.mli b/sledge/nonstdlib/option.mli
index 92cb6838c..9347d6aa8 100644
--- a/sledge/nonstdlib/option.mli
+++ b/sledge/nonstdlib/option.mli
@@ -17,4 +17,4 @@ val bind : 'a t -> f:('a -> 'b t) -> 'b t
 val iter : 'a t -> f:('a -> unit) -> unit
 val exists : 'a t -> f:('a -> bool) -> bool
 val for_all : 'a t -> f:('a -> bool) -> bool
-val fold : 'a t -> init:'s -> f:('s -> 'a -> 's) -> 's
+val fold : 'a t -> 's -> f:('a -> 's -> 's) -> 's
diff --git a/sledge/nonstdlib/set.ml b/sledge/nonstdlib/set.ml
index 5a29c7a92..2a2b2d70f 100644
--- a/sledge/nonstdlib/set.ml
+++ b/sledge/nonstdlib/set.ml
@@ -31,13 +31,13 @@ end) : S with type elt = Elt.t = struct
   let of_option xo = Option.map_or ~f:S.singleton xo ~default:empty
   let of_list = S.of_list
   let add x s = S.add x s
-  let add_option xo s = Option.fold ~f:(Fun.flip add) ~init:s xo
+  let add_option = Option.fold ~f:add
   let add_list xs s = S.add_list s xs
   let diff = S.diff
   let inter = S.inter
   let union = S.union
   let diff_inter s t = (diff s t, inter s t)
-  let union_list ss = List.fold ~f:union ~init:empty ss
+  let union_list ss = List.fold ~f:union ss empty
   let is_empty = S.is_empty
   let cardinal = S.cardinal
   let mem s x = S.mem x s
@@ -78,7 +78,7 @@ end) : S with type elt = Elt.t = struct
   let iter s ~f = S.iter f s
   let exists s ~f = S.exists f s
   let for_all s ~f = S.for_all f s
-  let fold s ~init ~f = S.fold f s init
+  let fold s z ~f = S.fold f s z
   let to_iter = S.to_iter
 
   let pp ?pre ?suf ?(sep = (",@ " : (unit, unit) fmt)) pp_elt fs x =
diff --git a/sledge/nonstdlib/set_intf.ml b/sledge/nonstdlib/set_intf.ml
index 87d381bb5..567cde947 100644
--- a/sledge/nonstdlib/set_intf.ml
+++ b/sledge/nonstdlib/set_intf.ml
@@ -55,7 +55,7 @@ module type S = sig
   val iter : t -> f:(elt -> unit) -> unit
   val exists : t -> f:(elt -> bool) -> bool
   val for_all : t -> f:(elt -> bool) -> bool
-  val fold : t -> init:'s -> f:(elt -> 's -> 's) -> 's
+  val fold : t -> 's -> f:(elt -> 's -> 's) -> 's
 
   (** {1 Convert} *)
 
diff --git a/sledge/report/sledge_report.ml b/sledge/report/sledge_report.ml
index 6e7429a4d..d20cb4107 100644
--- a/sledge/report/sledge_report.ml
+++ b/sledge/report/sledge_report.ml
@@ -41,12 +41,12 @@ let times_of_raw {Report.etime; utime; stime; cutime; cstime} =
   let etime = etime in
   {etime; utime; stime}
 
-let add_time base_times row ptimes =
+let add_time base_times ptimes row =
   let tustimes = times_of_raw ptimes in
   let times = tustimes :: row.times in
   let times_deltas =
-    Option.fold base_times ~init:row.times_deltas
-      ~f:(fun times_deltas {etime= btt; utime= but; stime= bst} ->
+    Option.fold base_times row.times_deltas
+      ~f:(fun {etime= btt; utime= but; stime= bst} times_deltas ->
         let {etime= tt; utime= ut; stime= st} = tustimes in
         {etime= tt -. btt; utime= ut -. but; stime= st -. bst}
         :: times_deltas )
@@ -56,12 +56,12 @@ let add_time base_times row ptimes =
 let add_times base_times times row =
   if List.is_empty times then
     {row with times_deltas= Option.to_list base_times}
-  else List.fold ~f:(add_time base_times) ~init:row times
+  else List.fold ~f:(add_time base_times) times row
 
-let add_gc base_gcs row gc =
+let add_gc base_gcs gc row =
   let gcs = gc :: row.gcs in
   let gcs_deltas =
-    Option.fold base_gcs ~init:row.gcs_deltas ~f:(fun gcs_deltas bgc ->
+    Option.fold base_gcs row.gcs_deltas ~f:(fun bgc gcs_deltas ->
         Report.
           { allocated= gc.allocated -. bgc.allocated
           ; promoted= gc.promoted -. bgc.promoted
@@ -72,9 +72,9 @@ let add_gc base_gcs row gc =
 
 let add_gcs base_gcs gcs row =
   if List.is_empty gcs then {row with gcs_deltas= Option.to_list base_gcs}
-  else List.fold ~f:(add_gc base_gcs) ~init:row gcs
+  else List.fold ~f:(add_gc base_gcs) gcs row
 
-let add_status base_status row status =
+let add_status base_status status row =
   if List.mem ~eq:Report.equal_status status row.status then row
   else
     match base_status with
@@ -88,13 +88,13 @@ let add_status base_status row status =
     | _ -> {row with status= status :: row.status}
 
 let add_statuses base_status statuses row =
-  List.fold ~f:(add_status base_status) ~init:row statuses
+  List.fold ~f:(add_status base_status) statuses row
 
 let ave_floats flts =
   assert (not (Iter.is_empty flts)) ;
   let min, max, sum, num =
-    Iter.fold flts ~init:(Float.infinity, Float.neg_infinity, 0., 0)
-      ~f:(fun (min, max, sum, num) flt ->
+    Iter.fold flts (Float.infinity, Float.neg_infinity, 0., 0)
+      ~f:(fun flt (min, max, sum, num) ->
         (Float.min min flt, Float.max max flt, sum +. flt, num + 1) )
   in
   if num >= 5 then (sum -. min -. max) /. Float.of_int (num - 2)
@@ -108,15 +108,12 @@ let combine name b_result c_result =
           if List.is_empty times then None
           else
             let etimes, utimes, stimes, cutimes, cstimes =
-              List.fold times
-                ~init:
-                  ( Iter.empty
-                  , Iter.empty
-                  , Iter.empty
-                  , Iter.empty
-                  , Iter.empty )
-                ~f:(fun (etimes, utimes, stimes, cutimes, cstimes)
-                   {Report.etime; utime; stime; cutime; cstime}
+              let init =
+                (Iter.empty, Iter.empty, Iter.empty, Iter.empty, Iter.empty)
+              in
+              List.fold times init
+                ~f:(fun {Report.etime; utime; stime; cutime; cstime}
+                   (etimes, utimes, stimes, cutimes, cstimes)
                    ->
                   ( Iter.cons etime etimes
                   , Iter.cons utime utimes
@@ -136,9 +133,9 @@ let combine name b_result c_result =
           if List.is_empty gcs then None
           else
             let allocs, promos, peaks =
-              List.fold gcs ~init:(Iter.empty, Iter.empty, Iter.empty)
-                ~f:(fun (allocs, promos, peaks)
-                   {Report.allocated; promoted; peak_size}
+              List.fold gcs (Iter.empty, Iter.empty, Iter.empty)
+                ~f:(fun {Report.allocated; promoted; peak_size}
+                   (allocs, promos, peaks)
                    ->
                   ( Iter.cons allocated allocs
                   , Iter.cons promoted promos
@@ -198,20 +195,20 @@ let ranges rows =
     ; max_peak= 0.
     ; pct_peak= 0. }
   in
-  Iter.fold rows ~init ~f:(fun acc {times; times_deltas; gcs; gcs_deltas} ->
-      Option.fold times_deltas ~init:acc ~f:(fun acc deltas ->
+  Iter.fold rows init ~f:(fun {times; times_deltas; gcs; gcs_deltas} acc ->
+      Option.fold times_deltas acc ~f:(fun deltas acc ->
           let max_time = Float.max acc.max_time (Float.abs deltas.etime) in
           let pct_time =
-            Option.fold times ~init:acc.pct_time ~f:(fun pct_time times ->
+            Option.fold times acc.pct_time ~f:(fun times pct_time ->
                 let pct = 100. *. deltas.etime /. times.etime in
                 Float.max pct_time (Float.abs pct) )
           in
           {acc with max_time; pct_time} )
-      |> fun init ->
-      Option.fold gcs_deltas ~init ~f:(fun acc deltas ->
+      |> fun acc ->
+      Option.fold gcs_deltas acc ~f:(fun deltas acc ->
           let max_alloc = Float.max acc.max_alloc deltas.Report.allocated in
           let pct_alloc =
-            Option.fold gcs ~init:acc.pct_alloc ~f:(fun pct_alloc gcs ->
+            Option.fold gcs acc.pct_alloc ~f:(fun gcs pct_alloc ->
                 let pct =
                   100. *. deltas.Report.allocated /. gcs.Report.allocated
                 in
@@ -219,7 +216,7 @@ let ranges rows =
           in
           let max_promo = Float.max acc.max_promo deltas.Report.promoted in
           let pct_promo =
-            Option.fold gcs ~init:acc.pct_promo ~f:(fun pct_promo gcs ->
+            Option.fold gcs acc.pct_promo ~f:(fun gcs pct_promo ->
                 let pct =
                   100. *. deltas.Report.promoted /. gcs.Report.promoted
                 in
@@ -227,7 +224,7 @@ let ranges rows =
           in
           let max_peak = Float.max acc.max_peak deltas.Report.peak_size in
           let pct_peak =
-            Option.fold gcs ~init:acc.pct_peak ~f:(fun pct_peak gcs ->
+            Option.fold gcs acc.pct_peak ~f:(fun gcs pct_peak ->
                 let pct =
                   100. *. deltas.Report.peak_size /. gcs.Report.peak_size
                 in
@@ -424,8 +421,8 @@ let average row =
     if List.is_empty times then None
     else
       let etimes, utimes, stimes =
-        List.fold times ~init:(Iter.empty, Iter.empty, Iter.empty)
-          ~f:(fun (etimes, utimes, stimes) {etime; utime; stime} ->
+        List.fold times (Iter.empty, Iter.empty, Iter.empty)
+          ~f:(fun {etime; utime; stime} (etimes, utimes, stimes) ->
             ( Iter.cons etime etimes
             , Iter.cons utime utimes
             , Iter.cons stime stimes ) )
@@ -441,9 +438,9 @@ let average row =
     if List.is_empty gcs then None
     else
       let alloc, promo, peak =
-        List.fold gcs ~init:(Iter.empty, Iter.empty, Iter.empty)
-          ~f:(fun (alloc, promo, peak)
-             {Report.allocated; promoted; peak_size}
+        List.fold gcs (Iter.empty, Iter.empty, Iter.empty)
+          ~f:(fun {Report.allocated; promoted; peak_size}
+             (alloc, promo, peak)
              ->
             ( Iter.cons allocated alloc
             , Iter.cons promoted promo
@@ -470,7 +467,7 @@ let add_total rows =
     ; status_deltas= None }
   in
   let total =
-    Iter.fold rows ~init ~f:(fun total row ->
+    Iter.fold rows init ~f:(fun total row ->
         let times =
           match (total.times, row.times) with
           | Some total_times, Some row_times ->
@@ -545,9 +542,7 @@ let input_rows ?baseline current =
   let names =
     let keys = Tbl.keys c_tbl in
     let keys =
-      Option.fold
-        ~f:(fun i t -> Iter.append (Tbl.keys t) i)
-        ~init:keys b_tbl
+      Option.fold ~f:(fun t -> Iter.append (Tbl.keys t)) b_tbl keys
     in
     Iter.sort_uniq ~cmp:String.compare keys
   in
diff --git a/sledge/src/arithmetic.ml b/sledge/src/arithmetic.ml
index bf07124a4..51f05f75f 100644
--- a/sledge/src/arithmetic.ml
+++ b/sledge/src/arithmetic.ml
@@ -242,10 +242,10 @@ module Representation (Trm : INDETERMINATE) = struct
     let map poly ~f =
       let p, p' = (poly, Sum.empty) in
       let p, p' =
-        Sum.fold poly ~init:(p, p') ~f:(fun mono coeff (p, p') ->
+        Sum.fold poly (p, p') ~f:(fun mono coeff (p, p') ->
             let m, cm' = (mono, CM.one) in
             let m, cm' =
-              Prod.fold mono ~init:(m, cm') ~f:(fun trm power (m, cm') ->
+              Prod.fold mono (m, cm') ~f:(fun trm power (m, cm') ->
                   let trm' = f trm in
                   if trm == trm' then (m, cm')
                   else
diff --git a/sledge/src/arithmetic_intf.ml b/sledge/src/arithmetic_intf.ml
index dea75ee4b..5f9759ec7 100644
--- a/sledge/src/arithmetic_intf.ml
+++ b/sledge/src/arithmetic_intf.ml
@@ -68,8 +68,8 @@ module type S = sig
 
   type product
 
-  val fold_factors : product -> init:'s -> f:(trm -> int -> 's -> 's) -> 's
-  val fold_monomials : t -> init:'s -> f:(product -> Q.t -> 's -> 's) -> 's
+  val fold_factors : product -> 's -> f:(trm -> int -> 's -> 's) -> 's
+  val fold_monomials : t -> 's -> f:(product -> Q.t -> 's -> 's) -> 's
 end
 
 (** Indeterminate terms, treated as atomic / variables except when they can
diff --git a/sledge/src/control.ml b/sledge/src/control.ml
index be926c8b9..3a0abe908 100644
--- a/sledge/src/control.ml
+++ b/sledge/src/control.ml
@@ -30,7 +30,7 @@ module Make (Dom : Domain_intf.Dom) = struct
 
     val pop_throw :
          t
-      -> init:'a
+      -> 'a
       -> unwind:
            (   Llair.Reg.t list
             -> Llair.Reg.Set.t
@@ -130,7 +130,7 @@ module Make (Dom : Domain_intf.Dom) = struct
       | Return {from_call; dst; stk} -> Some (from_call, dst, stk)
       | Empty -> None
 
-    let pop_throw stk ~init ~unwind =
+    let pop_throw stk state ~unwind =
       let rec pop_throw_ state = function
         | Return {formals; locals; from_call; stk} ->
             pop_throw_ (unwind formals locals from_call state) stk
@@ -139,7 +139,7 @@ module Make (Dom : Domain_intf.Dom) = struct
         | Empty -> None
         | Throw _ as stk -> violates invariant stk
       in
-      pop_throw_ init stk
+      pop_throw_ state stk
   end
 
   module Work : sig
@@ -240,7 +240,7 @@ module Make (Dom : Domain_intf.Dom) = struct
         | Some (q :: qs, ws) ->
             let join (qa, da) (q, d) = (Dom.join q qa, Depths.join d da) in
             let skipped, (qs, depths) =
-              List.fold qs ~init:([], q) ~f:(fun (skipped, joined) curr ->
+              List.fold qs ([], q) ~f:(fun curr (skipped, joined) ->
                   match join curr joined with
                   | Some joined, depths -> (skipped, (joined, depths))
                   | None, _ -> (curr :: skipped, joined) )
@@ -275,7 +275,7 @@ module Make (Dom : Domain_intf.Dom) = struct
     let domain_call =
       Dom.call ~globals ~actuals ~areturn ~formals ~freturn ~locals
     in
-    List.fold ~init:Work.skip dnf_states ~f:(fun acc state ->
+    List.fold dnf_states Work.skip ~f:(fun state acc ->
         match
           if not opts.function_summaries then None
           else
@@ -337,7 +337,7 @@ module Make (Dom : Domain_intf.Dom) = struct
     let exit_state =
       match (freturn, exp) with
       | Some freturn, Some return_val ->
-          Dom.exec_move pre_state (IArray.of_ (freturn, return_val))
+          Dom.exec_move (IArray.of_ (freturn, return_val)) pre_state
       | None, None -> pre_state
       | _ -> violates Llair.Func.invariant block.parent
     in
@@ -365,11 +365,11 @@ module Make (Dom : Domain_intf.Dom) = struct
       Dom.retn formals (Some func.fthrow) from_call
         (Dom.post scope from_call state)
     in
-    ( match Stack.pop_throw stk ~unwind ~init:pre_state with
+    ( match Stack.pop_throw stk ~unwind pre_state with
     | Some (from_call, retn_site, stk, unwind_state) ->
         let fthrow = func.fthrow in
         let exit_state =
-          Dom.exec_move unwind_state (IArray.of_ (fthrow, exc))
+          Dom.exec_move (IArray.of_ (fthrow, exc)) unwind_state
         in
         let post_state = Dom.post func.locals from_call exit_state in
         let retn_state =
@@ -389,7 +389,7 @@ module Make (Dom : Domain_intf.Dom) = struct
       -> Work.x =
    fun stk state block areturn return ->
     Report.unknown_call block.term ;
-    let state = Option.fold ~f:Dom.exec_kill ~init:state areturn in
+    let state = Option.fold ~f:Dom.exec_kill areturn state in
     exec_jump stk state block return
 
   let exec_term :
@@ -405,22 +405,21 @@ module Make (Dom : Domain_intf.Dom) = struct
     Report.step () ;
     match block.term with
     | Switch {key; tbl; els} ->
-        IArray.fold tbl
-          ~f:(fun x (case, jump) ->
+        IArray.fold
+          ~f:(fun (case, jump) x ->
             match Dom.exec_assume state (Llair.Exp.eq key case) with
             | Some state -> exec_jump stk state block jump |> Work.seq x
             | None -> x )
-          ~init:
-            ( match
-                Dom.exec_assume state
-                  (IArray.fold tbl ~init:Llair.Exp.true_
-                     ~f:(fun b (case, _) ->
-                       Llair.Exp.and_ (Llair.Exp.dq key case) b ))
-              with
-            | Some state -> exec_jump stk state block els
-            | None -> Work.skip )
+          tbl
+          ( match
+              Dom.exec_assume state
+                (IArray.fold tbl Llair.Exp.true_ ~f:(fun (case, _) b ->
+                     Llair.Exp.and_ (Llair.Exp.dq key case) b ))
+            with
+          | Some state -> exec_jump stk state block els
+          | None -> Work.skip )
     | Iswitch {ptr; tbl} ->
-        IArray.fold tbl ~init:Work.skip ~f:(fun x (jump : Llair.jump) ->
+        IArray.fold tbl Work.skip ~f:(fun (jump : Llair.jump) x ->
             match
               Dom.exec_assume state
                 (Llair.Exp.eq ptr
@@ -438,10 +437,10 @@ module Make (Dom : Domain_intf.Dom) = struct
         match callees with
         | [] -> exec_skip_func stk state block areturn return
         | callees ->
-            List.fold callees ~init:Work.skip ~f:(fun x callee ->
+            List.fold callees Work.skip ~f:(fun callee x ->
                 ( match
-                    Dom.exec_intrinsic ~skip_throw:opts.skip_throw state
-                      areturn callee.name.reg actuals
+                    Dom.exec_intrinsic ~skip_throw:opts.skip_throw areturn
+                      callee.name.reg actuals state
                   with
                 | Some None ->
                     Report.invalid_access_term
@@ -463,13 +462,13 @@ module Make (Dom : Domain_intf.Dom) = struct
         else exec_throw stk state block exc
     | Unreachable -> Work.skip
 
-  let exec_inst : Dom.t -> Llair.inst -> (Dom.t, Dom.t * Llair.inst) result
+  let exec_inst : Llair.inst -> Dom.t -> (Dom.t, Dom.t * Llair.inst) result
       =
-   fun state inst ->
+   fun inst state ->
     [%Trace.info
       "@[<2>exec inst@\n@[%a@]@\n%a@]" Dom.pp state Llair.Inst.pp inst] ;
     Report.step () ;
-    Dom.exec_inst state inst
+    Dom.exec_inst inst state
     |> function
     | Some state -> Result.Ok state | None -> Result.Error (state, inst)
 
@@ -483,7 +482,7 @@ module Make (Dom : Domain_intf.Dom) = struct
    fun opts pgm stk state block ->
     [%Trace.info "exec block %%%s" block.lbl] ;
     match
-      Iter.fold_result ~f:exec_inst ~init:state (IArray.to_iter block.cmnd)
+      Iter.fold_result ~f:exec_inst (IArray.to_iter block.cmnd) state
     with
     | Ok state -> exec_term opts pgm stk state block
     | Error (state, inst) ->
@@ -517,7 +516,6 @@ module Make (Dom : Domain_intf.Dom) = struct
   let compute_summaries opts pgm : Dom.summary list Llair.Reg.Map.t =
     assert opts.function_summaries ;
     exec_pgm opts pgm ;
-    RegTbl.fold summary_table ~init:Llair.Reg.Map.empty
-      ~f:(fun ~key ~data map ->
+    RegTbl.fold summary_table Llair.Reg.Map.empty ~f:(fun ~key ~data map ->
         match data with [] -> map | _ -> Llair.Reg.Map.add ~key ~data map )
 end
diff --git a/sledge/src/domain_intf.ml b/sledge/src/domain_intf.ml
index 3e5664116..65fdaa8e1 100644
--- a/sledge/src/domain_intf.ml
+++ b/sledge/src/domain_intf.ml
@@ -16,16 +16,16 @@ module type Dom = sig
   val is_false : t -> bool
   val dnf : t -> t list
   val exec_assume : t -> Llair.Exp.t -> t option
-  val exec_kill : t -> Llair.Reg.t -> t
-  val exec_move : t -> (Llair.Reg.t * Llair.Exp.t) iarray -> t
-  val exec_inst : t -> Llair.inst -> t option
+  val exec_kill : Llair.Reg.t -> t -> t
+  val exec_move : (Llair.Reg.t * Llair.Exp.t) iarray -> t -> t
+  val exec_inst : Llair.inst -> t -> t option
 
   val exec_intrinsic :
        skip_throw:bool
-    -> t
     -> Llair.Reg.t option
     -> Llair.Reg.t
     -> Llair.Exp.t list
+    -> t
     -> t option option
 
   type from_call [@@deriving sexp_of]
diff --git a/sledge/src/domain_relation.ml b/sledge/src/domain_relation.ml
index 3f5069cb1..7caad0b76 100644
--- a/sledge/src/domain_relation.ml
+++ b/sledge/src/domain_relation.ml
@@ -45,21 +45,21 @@ module Make (State_domain : State_domain_sig) = struct
     let+ next = State_domain.exec_assume current cnd in
     (entry, next)
 
-  let exec_kill (entry, current) reg =
-    (entry, State_domain.exec_kill current reg)
+  let exec_kill reg (entry, current) =
+    (entry, State_domain.exec_kill reg current)
 
-  let exec_move (entry, current) reg_exps =
-    (entry, State_domain.exec_move current reg_exps)
+  let exec_move reg_exps (entry, current) =
+    (entry, State_domain.exec_move reg_exps current)
 
-  let exec_inst (entry, current) inst =
-    let+ next = State_domain.exec_inst current inst in
+  let exec_inst inst (entry, current) =
+    let+ next = State_domain.exec_inst inst current in
     (entry, next)
 
-  let exec_intrinsic ~skip_throw (entry, current) areturn intrinsic actuals
+  let exec_intrinsic ~skip_throw areturn intrinsic actuals (entry, current)
       =
     let+ next_opt =
-      State_domain.exec_intrinsic ~skip_throw current areturn intrinsic
-        actuals
+      State_domain.exec_intrinsic ~skip_throw areturn intrinsic actuals
+        current
     in
     let+ next = next_opt in
     (entry, next)
diff --git a/sledge/src/domain_sh.ml b/sledge/src/domain_sh.ml
index 0f6667cc4..7ec996f3b 100644
--- a/sledge/src/domain_sh.ml
+++ b/sledge/src/domain_sh.ml
@@ -20,13 +20,14 @@ let simplify_states = ref true
 let simplify q = if !simplify_states then Sh.simplify q else q
 
 let init globals =
-  IArray.fold globals ~init:Sh.emp ~f:(fun q -> function
-    | {Llair.Global.reg; init= Some (seq, siz)} ->
-        let loc = Term.var (X.reg reg) in
-        let len = Term.integer (Z.of_int siz) in
-        let seq = X.term seq in
-        Sh.star q (Sh.seg {loc; bas= loc; len; siz= len; seq})
-    | _ -> q )
+  IArray.fold globals Sh.emp ~f:(fun global q ->
+      match global with
+      | {Llair.Global.reg; init= Some (seq, siz)} ->
+          let loc = Term.var (X.reg reg) in
+          let len = Term.integer (Z.of_int siz) in
+          let seq = X.term seq in
+          Sh.star q (Sh.seg {loc; bas= loc; len; siz= len; seq})
+      | _ -> q )
 
 let join p q =
   [%Trace.call fun {pf} -> pf "%a@ %a" pp p pp q]
@@ -38,13 +39,13 @@ let join p q =
 let is_false = Sh.is_false
 let dnf = Sh.dnf
 let exec_assume q b = Exec.assume q (X.formula b) |> Option.map ~f:simplify
-let exec_kill q r = Exec.kill q (X.reg r) |> simplify
+let exec_kill r q = Exec.kill q (X.reg r) |> simplify
 
-let exec_move q res =
+let exec_move res q =
   Exec.move q (IArray.map res ~f:(fun (r, e) -> (X.reg r, X.term e)))
   |> simplify
 
-let exec_inst pre inst =
+let exec_inst inst pre =
   ( match (inst : Llair.inst) with
   | Move {reg_exps; _} ->
       Some
@@ -67,7 +68,7 @@ let exec_inst pre inst =
   | Abort _ -> Exec.abort pre )
   |> Option.map ~f:simplify
 
-let exec_intrinsic ~skip_throw q r i es =
+let exec_intrinsic ~skip_throw r i es q =
   Exec.intrinsic ~skip_throw q (Option.map ~f:X.reg r) (X.reg i)
     (List.map ~f:X.term es)
   |> Option.map ~f:(Option.map ~f:simplify)
@@ -94,10 +95,10 @@ let garbage_collect (q : t) ~wrt =
     if Var.Set.equal previous current then current
     else
       let new_set =
-        List.fold ~init:current q.heap ~f:(fun current seg ->
+        List.fold q.heap current ~f:(fun seg current ->
             if term_eq_class_has_only_vars_in current q.ctx seg.loc then
-              List.fold (Context.class_of q.ctx seg.seq) ~init:current
-                ~f:(fun c e -> Var.Set.union c (Term.fv e))
+              List.fold (Context.class_of q.ctx seg.seq) current
+                ~f:(fun e c -> Var.Set.union c (Term.fv e))
             else current )
       in
       all_reachable_vars current new_set q
@@ -109,11 +110,11 @@ let garbage_collect (q : t) ~wrt =
   [%Trace.retn fun {pf} -> pf "%a" pp]
 
 let and_eqs sub formals actuals q =
-  let and_eq q formal actual =
+  let and_eq formal actual q =
     let actual' = Term.rename sub actual in
     Sh.and_ (Formula.eq (Term.var formal) actual') q
   in
-  List.fold2_exn ~f:and_eq formals actuals ~init:q
+  List.fold2_exn ~f:and_eq formals actuals q
 
 let localize_entry globals actuals formals freturn locals shadow pre entry =
   (* Add the formals here to do garbage collection and then get rid of them *)
@@ -257,7 +258,7 @@ let create_summary ~locals ~formals ~entry ~current:(post : Sh.t) =
   let foot = Sh.exists locals entry in
   let foot, subst = Sh.freshen ~wrt:(Var.Set.union foot.us post.us) foot in
   let restore_formals q =
-    Var.Set.fold formals ~init:q ~f:(fun var q ->
+    Var.Set.fold formals q ~f:(fun var q ->
         let var = Term.var var in
         let renamed_var = Term.rename subst var in
         Sh.and_ (Formula.eq renamed_var var) q )
diff --git a/sledge/src/domain_unit.ml b/sledge/src/domain_unit.ml
index eb46f7372..fd494b79d 100644
--- a/sledge/src/domain_unit.ml
+++ b/sledge/src/domain_unit.ml
@@ -15,9 +15,9 @@ let init _ = ()
 let join () () = Some ()
 let is_false _ = false
 let exec_assume () _ = Some ()
-let exec_kill () _ = ()
-let exec_move () _ = ()
-let exec_inst () _ = Some ()
+let exec_kill _ () = ()
+let exec_move _ () = ()
+let exec_inst _ () = Some ()
 let exec_intrinsic ~skip_throw:_ _ _ _ _ : t option option = None
 
 type from_call = unit [@@deriving compare, equal, sexp]
diff --git a/sledge/src/domain_used_globals.ml b/sledge/src/domain_used_globals.ml
index 3764ea3da..82be51b26 100644
--- a/sledge/src/domain_used_globals.ml
+++ b/sledge/src/domain_used_globals.ml
@@ -24,30 +24,25 @@ let post _ _ state = state
 let retn _ _ from_call post = Llair.Reg.Set.union from_call post
 let dnf t = [t]
 
-let add_if_global gs v =
+let add_if_global v gs =
   if Llair.Reg.is_global v then Llair.Reg.Set.add v gs else gs
 
-let used_globals ?(init = empty) exp =
-  Llair.Exp.fold_regs exp ~init ~f:add_if_global
+let used_globals exp s = Llair.Exp.fold_regs ~f:add_if_global exp s
+let exec_assume st exp = Some (used_globals exp st)
+let exec_kill _ st = st
 
-let exec_assume st exp = Some (used_globals ~init:st exp)
-let exec_kill st _ = st
+let exec_move reg_exps st =
+  IArray.fold ~f:(fun (_, rhs) -> used_globals rhs) reg_exps st
 
-let exec_move st reg_exps =
-  IArray.fold reg_exps ~init:st ~f:(fun st (_, rhs) ->
-      used_globals ~init:st rhs )
-
-let exec_inst st inst =
+let exec_inst inst st =
   [%Trace.call fun {pf} -> pf "pre:{%a} %a" pp st Llair.Inst.pp inst]
   ;
-  Some
-    (Llair.Inst.fold_exps inst ~init:st ~f:(fun acc e ->
-         used_globals ~init:acc e ))
+  Some (Llair.Inst.fold_exps ~f:used_globals inst st)
   |>
   [%Trace.retn fun {pf} ->
     Option.iter ~f:(fun uses -> pf "post:{%a}" pp uses)]
 
-let exec_intrinsic ~skip_throw:_ st _ intrinsic actuals =
+let exec_intrinsic ~skip_throw:_ _ intrinsic actuals st =
   let name = Llair.Reg.name intrinsic in
   if
     List.exists
@@ -72,9 +67,7 @@ let exec_intrinsic ~skip_throw:_ st _ intrinsic actuals =
       ; "__cxa_allocate_exception"
       ; "_ZN5folly13usingJEMallocEv" ]
       ~f:(String.equal name)
-  then
-    List.fold actuals ~init:st ~f:(fun s a -> used_globals ~init:s a)
-    |> fun res -> Some (Some res)
+  then List.fold ~f:used_globals actuals st |> fun res -> Some (Some res)
   else None
 
 type from_call = t [@@deriving sexp]
@@ -82,10 +75,10 @@ type from_call = t [@@deriving sexp]
 (* Set abstract state to bottom (i.e. empty set) at function entry *)
 let call ~summaries:_ ~globals:_ ~actuals ~areturn:_ ~formals:_ ~freturn:_
     ~locals:_ st =
-  (empty, List.fold actuals ~init:st ~f:(fun s a -> used_globals ~init:s a))
+  (empty, List.fold ~f:used_globals actuals st)
 
 let resolve_callee lookup ptr st =
-  let st = used_globals ~init:st ptr in
+  let st = used_globals ptr st in
   match Llair.Reg.of_exp ptr with
   | Some callee -> (lookup (Llair.Reg.name callee), st)
   | None -> ([], st)
diff --git a/sledge/src/exec.ml b/sledge/src/exec.ml
index df71c2f25..569ea240b 100644
--- a/sledge/src/exec.ml
+++ b/sledge/src/exec.ml
@@ -102,13 +102,13 @@ open Fresh.Import
 let move_spec reg_exps =
   let foot = Sh.emp in
   let ws, rs =
-    IArray.fold reg_exps ~init:(Var.Set.empty, Var.Set.empty)
-      ~f:(fun (ws, rs) (reg, exp) ->
+    IArray.fold reg_exps (Var.Set.empty, Var.Set.empty)
+      ~f:(fun (reg, exp) (ws, rs) ->
         (Var.Set.add reg ws, Var.Set.union rs (Term.fv exp)) )
   in
   let+ sub, ms = Fresh.assign ~ws ~rs in
   let post =
-    IArray.fold reg_exps ~init:Sh.emp ~f:(fun post (reg, exp) ->
+    IArray.fold reg_exps Sh.emp ~f:(fun (reg, exp) post ->
         Sh.and_ (Formula.eq (Term.var reg) (Term.rename sub exp)) post )
   in
   {foot; sub; ms; post}
diff --git a/sledge/src/fol.ml b/sledge/src/fol.ml
index 6925954e4..a1d4f9648 100644
--- a/sledge/src/fol.ml
+++ b/sledge/src/fol.ml
@@ -350,51 +350,46 @@ let pp = ppx (fun _ -> None)
 
 (** fold_vars *)
 
-let fold_pos_neg ~pos ~neg ~init ~f =
-  let f_not p s = f s (_Not p) in
-  Fmls.fold ~init:(Fmls.fold ~init ~f:(Fun.flip f) pos) ~f:f_not neg
+let fold_pos_neg ~pos ~neg s ~f =
+  let f_not p s = f (_Not p) s in
+  Fmls.fold ~f:f_not neg (Fmls.fold ~f pos s)
 
-let rec fold_vars_t e ~init ~f =
+let rec fold_vars_t e s ~f =
   match e with
-  | Z _ | Q _ | Ancestor _ -> init
-  | Var _ as v -> f init (Var.of_ v)
-  | Splat x | Select {rcd= x} -> fold_vars_t ~f x ~init
+  | Z _ | Q _ | Ancestor _ -> s
+  | Var _ as v -> f (Var.of_ v) s
+  | Splat x | Select {rcd= x} -> fold_vars_t ~f x s
   | Sized {seq= x; siz= y} | Update {rcd= x; elt= y} ->
-      fold_vars_t ~f x ~init:(fold_vars_t ~f y ~init)
+      fold_vars_t ~f x (fold_vars_t ~f y s)
   | Extract {seq= x; off= y; len= z} ->
-      fold_vars_t ~f x
-        ~init:(fold_vars_t ~f y ~init:(fold_vars_t ~f z ~init))
+      fold_vars_t ~f x (fold_vars_t ~f y (fold_vars_t ~f z s))
   | Concat xs | Record xs | Apply (_, xs) ->
-      Array.fold ~f:(fun init -> fold_vars_t ~f ~init) xs ~init
-  | Arith a ->
-      Iter.fold
-        ~f:(fun s x -> fold_vars_t ~f x ~init:s)
-        ~init (Arith.iter a)
+      Array.fold ~f:(fold_vars_t ~f) xs s
+  | Arith a -> Iter.fold ~f:(fold_vars_t ~f) (Arith.iter a) s
 
-let rec fold_vars_f ~init p ~f =
+let rec fold_vars_f p s ~f =
   match (p : fml) with
-  | Tt -> init
-  | Eq (x, y) -> fold_vars_t ~f x ~init:(fold_vars_t ~f y ~init)
-  | Eq0 x | Pos x -> fold_vars_t ~f x ~init
-  | Not x -> fold_vars_f ~f x ~init
+  | Tt -> s
+  | Eq (x, y) -> fold_vars_t ~f x (fold_vars_t ~f y s)
+  | Eq0 x | Pos x -> fold_vars_t ~f x s
+  | Not x -> fold_vars_f ~f x s
   | And {pos; neg} | Or {pos; neg} ->
-      fold_pos_neg ~f:(fun init -> fold_vars_f ~f ~init) ~pos ~neg ~init
-  | Iff (x, y) -> fold_vars_f ~f x ~init:(fold_vars_f ~f y ~init)
+      fold_pos_neg ~f:(fold_vars_f ~f) ~pos ~neg s
+  | Iff (x, y) -> fold_vars_f ~f x (fold_vars_f ~f y s)
   | Cond {cnd; pos; neg} ->
-      fold_vars_f ~f cnd
-        ~init:(fold_vars_f ~f pos ~init:(fold_vars_f ~f neg ~init))
-  | Lit (_, xs) -> Array.fold ~f:(fun init -> fold_vars_t ~f ~init) xs ~init
+      fold_vars_f ~f cnd (fold_vars_f ~f pos (fold_vars_f ~f neg s))
+  | Lit (_, xs) -> Array.fold ~f:(fold_vars_t ~f) xs s
 
-let rec fold_vars_c ~init ~f = function
+let rec fold_vars_c c s ~f =
+  match c with
   | `Ite (cnd, thn, els) ->
-      fold_vars_f ~f cnd
-        ~init:(fold_vars_c ~f thn ~init:(fold_vars_c ~f els ~init))
-  | `Trm t -> fold_vars_t ~f t ~init
+      fold_vars_f ~f cnd (fold_vars_c ~f thn (fold_vars_c ~f els s))
+  | `Trm t -> fold_vars_t ~f t s
 
-let fold_vars ~init e ~f =
+let fold_vars e s ~f =
   match e with
-  | `Fml p -> fold_vars_f ~f ~init p
-  | #cnd as c -> fold_vars_c ~f ~init c
+  | `Fml p -> fold_vars_f ~f p s
+  | #cnd as c -> fold_vars_c ~f c s
 
 (** map *)
 
@@ -704,21 +699,21 @@ module Term = struct
 
   let map_vars = map_vars
 
-  let fold_map_vars e ~init ~f =
-    let s = ref init in
+  let fold_map_vars e s0 ~f =
+    let s = ref s0 in
     let f x =
-      let s', x' = f !s x in
+      let x', s' = f x !s in
       s := s' ;
       x'
     in
     let e' = map_vars ~f e in
-    (!s, e')
+    (e', !s)
 
   let rename s e = map_vars ~f:(Var.Subst.apply s) e
 
   (** Query *)
 
-  let fv e = fold_vars e ~f:(Fun.flip Var.Set.add) ~init:Var.Set.empty
+  let fv e = fold_vars ~f:Var.Set.add e Var.Set.empty
 end
 
 (*
@@ -763,9 +758,9 @@ module Formula = struct
   (* connectives *)
 
   let and_ = and_
-  let andN = function [] -> tt | b :: bs -> List.fold ~init:b ~f:and_ bs
+  let andN = function [] -> tt | b :: bs -> List.fold ~f:and_ bs b
   let or_ = or_
-  let orN = function [] -> ff | b :: bs -> List.fold ~init:b ~f:or_ bs
+  let orN = function [] -> ff | b :: bs -> List.fold ~f:or_ bs b
   let iff = _Iff
   let xor p q = _Not (_Iff p q)
   let cond ~cnd ~pos ~neg = _Cond cnd pos neg
@@ -773,7 +768,7 @@ module Formula = struct
 
   (** Query *)
 
-  let fv e = fold_vars_f e ~f:(Fun.flip Var.Set.add) ~init:Var.Set.empty
+  let fv e = fold_vars_f ~f:Var.Set.add e Var.Set.empty
 
   (** Traverse *)
 
@@ -808,15 +803,15 @@ module Formula = struct
     | Cond {cnd; pos; neg} -> map3 (map_terms ~f) b _Cond cnd pos neg
     | Lit (p, xs) -> lift_mapN f b (_Lit p) xs
 
-  let fold_map_vars ~init e ~f =
-    let s = ref init in
+  let fold_map_vars e s0 ~f =
+    let s = ref s0 in
     let f x =
-      let s', x' = f !s x in
+      let x', s' = f x !s in
       s := s' ;
       x'
     in
     let e' = map_vars ~f e in
-    (!s, e')
+    (e', !s)
 
   let rename s e = map_vars ~f:(Var.Subst.apply s) e
 
@@ -828,25 +823,26 @@ module Formula = struct
       -> 'formula
       -> 'disjunction =
    fun ~meet1 ~join1 ~top ~bot fml ->
-    let rec add_conjunct (cjn, splits) fml =
+    let rec add_conjunct fml (cjn, splits) =
       match fml with
       | Tt | Eq _ | Eq0 _ | Pos _ | Iff _ | Lit _ | Not _ ->
           (meet1 fml cjn, splits)
       | And {pos; neg} ->
-          fold_pos_neg ~f:add_conjunct ~init:(cjn, splits) ~pos ~neg
+          fold_pos_neg ~f:add_conjunct ~pos ~neg (cjn, splits)
       | Or {pos; neg} -> (cjn, (pos, neg) :: splits)
       | Cond {cnd; pos; neg} ->
-          add_conjunct (cjn, splits)
+          add_conjunct
             (or_ (and_ cnd pos) (and_ (not_ cnd) neg))
+            (cjn, splits)
     in
-    let rec add_disjunct (cjn, splits) djn fml =
-      let cjn, splits = add_conjunct (cjn, splits) fml in
+    let rec add_disjunct (cjn, splits) fml djn =
+      let cjn, splits = add_conjunct fml (cjn, splits) in
       match splits with
       | (pos, neg) :: splits ->
-          fold_pos_neg ~f:(add_disjunct (cjn, splits)) ~init:djn ~pos ~neg
+          fold_pos_neg ~f:(add_disjunct (cjn, splits)) ~pos ~neg djn
       | [] -> join1 cjn djn
     in
-    add_disjunct (top, []) bot fml
+    add_disjunct (top, []) fml bot
 end
 
 (*
@@ -858,14 +854,14 @@ let v_to_ses : var -> Ses.Var.t =
 
 let vs_to_ses : Var.Set.t -> Ses.Var.Set.t =
  fun vs ->
-  Var.Set.fold vs ~init:Ses.Var.Set.empty ~f:(fun v vs ->
-      Ses.Var.Set.add (v_to_ses v) vs )
+  Var.Set.fold vs Ses.Var.Set.empty ~f:(fun v ->
+      Ses.Var.Set.add (v_to_ses v) )
 
 let rec arith_to_ses poly =
-  Arith.fold_monomials poly ~init:Ses.Term.zero ~f:(fun mono coeff e ->
+  Arith.fold_monomials poly Ses.Term.zero ~f:(fun mono coeff e ->
       Ses.Term.add e
         (Ses.Term.mulq coeff
-           (Arith.fold_factors mono ~init:Ses.Term.one ~f:(fun trm pow f ->
+           (Arith.fold_factors mono Ses.Term.one ~f:(fun trm pow f ->
                 let rec exp b i =
                   assert (i > 0) ;
                   if i = 1 then b else Ses.Term.mul b (exp f (i - 1))
@@ -911,12 +907,12 @@ let rec f_to_ses : fml -> Ses.Term.t = function
   | Not p -> Ses.Term.not_ (f_to_ses p)
   | And {pos; neg} ->
       fold_pos_neg
-        ~f:(fun p f -> Ses.Term.and_ p (f_to_ses f))
-        ~init:Ses.Term.true_ ~pos ~neg
+        ~f:(fun f p -> Ses.Term.and_ p (f_to_ses f))
+        ~pos ~neg Ses.Term.true_
   | Or {pos; neg} ->
       fold_pos_neg
-        ~f:(fun p f -> Ses.Term.or_ p (f_to_ses f))
-        ~init:Ses.Term.false_ ~pos ~neg
+        ~f:(fun f p -> Ses.Term.or_ p (f_to_ses f))
+        ~pos ~neg Ses.Term.false_
   | Iff (p, q) -> Ses.Term.eq (f_to_ses p) (f_to_ses q)
   | Cond {cnd; pos; neg} ->
       Ses.Term.conditional ~cnd:(f_to_ses cnd) ~thn:(f_to_ses pos)
@@ -941,8 +937,7 @@ let v_of_ses : Ses.Var.t -> var =
 
 let vs_of_ses : Ses.Var.Set.t -> Var.Set.t =
  fun vs ->
-  Ses.Var.Set.fold vs ~init:Var.Set.empty ~f:(fun v vs ->
-      Var.Set.add (v_of_ses v) vs )
+  Ses.Var.Set.fold ~f:(fun v -> Var.Set.add (v_of_ses v)) vs Var.Set.empty
 
 let uap1 f = ap1t (fun x -> _Apply f [|x|])
 let uap2 f = ap2t (fun x y -> _Apply f [|x; y|])
@@ -960,7 +955,7 @@ and ap2_f mk_f mk_t a b = ap2 mk_f (fun x y -> `Fml (mk_t x y)) a b
 
 and apN mk_f mk_t mk_unit es =
   match
-    Ses.Term.Set.fold ~init:(None, None) es ~f:(fun e (fs, ts) ->
+    Ses.Term.Set.fold es (None, None) ~f:(fun e (fs, ts) ->
         match of_ses e with
         | `Fml f ->
             (Some (match fs with None -> f | Some g -> mk_f f g), ts)
@@ -1001,8 +996,7 @@ and of_ses : Ses.Term.t -> exp =
             | `Trm (Q r) -> rational (Q.mul q r)
             | t -> mulq q t
         in
-        Ses.Term.Qset.fold sum ~init:(mul e q) ~f:(fun e q s ->
-            add (mul e q) s ) )
+        Ses.Term.Qset.fold ~f:(fun e q -> add (mul e q)) sum (mul e q) )
   | Mul prod -> (
     match Ses.Term.Qset.pop_min_elt prod with
     | None -> one
@@ -1018,8 +1012,7 @@ and of_ses : Ses.Term.t -> exp =
           else if sn > 0 then expn (of_ses e) n
           else div one (expn (of_ses e) (Z.neg n))
         in
-        Ses.Term.Qset.fold prod ~init:(exp e q) ~f:(fun e q s ->
-            mul (exp e q) s ) )
+        Ses.Term.Qset.fold ~f:(fun e q -> mul (exp e q)) prod (exp e q) )
   | Ap2 (Div, d, e) -> div (of_ses d) (of_ses e)
   | Ap2 (Rem, d, e) -> uap_ttt Rem d e
   | And es -> apN and_ (uap2 BitAnd) tt es
@@ -1073,10 +1066,10 @@ module Context = struct
 
   (* Query *)
 
-  let fold_vars ~init x ~f =
-    Ses.Equality.fold_vars x ~init ~f:(fun s v -> f s (v_of_ses v))
+  let fold_vars x s ~f =
+    Ses.Equality.fold_vars ~f:(fun v -> f (v_of_ses v)) x s
 
-  let fv e = fold_vars e ~f:(Fun.flip Var.Set.add) ~init:Var.Set.empty
+  let fv e = fold_vars ~f:Var.Set.add e Var.Set.empty
   let is_empty x = Ses.Equality.is_true x
   let is_unsat x = Ses.Equality.is_false x
   let implies x b = Ses.Equality.implies x (f_to_ses b)
@@ -1091,7 +1084,7 @@ module Context = struct
   let class_of x e = List.map ~f:of_ses (Ses.Equality.class_of x (to_ses e))
 
   let classes x =
-    Ses.Term.Map.fold (Ses.Equality.classes x) ~init:Term.Map.empty
+    Ses.Term.Map.fold (Ses.Equality.classes x) Term.Map.empty
       ~f:(fun ~key:rep ~data:cls clss ->
         let rep' = of_ses rep in
         let cls' = List.map ~f:of_ses cls in
@@ -1178,8 +1171,8 @@ module Context = struct
     let pp = Ses.Equality.Subst.pp
     let is_empty = Ses.Equality.Subst.is_empty
 
-    let fold s ~init ~f =
-      Ses.Equality.Subst.fold s ~init ~f:(fun ~key ~data ->
+    let fold s z ~f =
+      Ses.Equality.Subst.fold s z ~f:(fun ~key ~data ->
           f ~key:(of_ses key) ~data:(of_ses data) )
 
     let subst s = ses_map (Ses.Equality.Subst.subst s)
diff --git a/sledge/src/fol.mli b/sledge/src/fol.mli
index ae83a148c..2ffcd4106 100644
--- a/sledge/src/fol.mli
+++ b/sledge/src/fol.mli
@@ -76,15 +76,12 @@ module rec Term : sig
 
   (** Traverse *)
 
-  val fold_vars : init:'a -> t -> f:('a -> Var.t -> 'a) -> 'a
+  val fold_vars : t -> 's -> f:(Var.t -> 's -> 's) -> 's
 
   (** Transform *)
 
   val map_vars : f:(Var.t -> Var.t) -> t -> t
-
-  val fold_map_vars :
-    t -> init:'a -> f:('a -> Var.t -> 'a * Var.t) -> 'a * t
-
+  val fold_map_vars : t -> 's -> f:(Var.t -> 's -> Var.t * 's) -> t * 's
   val rename : Var.Subst.t -> t -> t
 end
 
@@ -137,16 +134,13 @@ and Formula : sig
 
   (** Traverse *)
 
-  val fold_vars : init:'a -> t -> f:('a -> Var.t -> 'a) -> 'a
+  val fold_vars : t -> 's -> f:(Var.t -> 's -> 's) -> 's
 
   (** Transform *)
 
   val map_terms : f:(Term.t -> Term.t) -> t -> t
   val map_vars : f:(Var.t -> Var.t) -> t -> t
-
-  val fold_map_vars :
-    init:'a -> t -> f:('a -> Var.t -> 'a * Var.t) -> 'a * t
-
+  val fold_map_vars : t -> 's -> f:(Var.t -> 's -> Var.t * 's) -> t * 's
   val rename : Var.Subst.t -> t -> t
 end
 
@@ -209,7 +203,7 @@ module Context : sig
   (** Equivalence class of [e]: all the terms [f] in the context such that
       [e = f] is implied by the assumptions. *)
 
-  val fold_vars : init:'a -> t -> f:('a -> Var.t -> 'a) -> 'a
+  val fold_vars : t -> 's -> f:(Var.t -> 's -> 's) -> 's
   (** Enumerate the variables occurring in the terms of the context. *)
 
   val fv : t -> Var.Set.t
@@ -221,9 +215,7 @@ module Context : sig
 
     val pp : t pp
     val is_empty : t -> bool
-
-    val fold :
-      t -> init:'a -> f:(key:Term.t -> data:Term.t -> 'a -> 'a) -> 'a
+    val fold : t -> 's -> f:(key:Term.t -> data:Term.t -> 's -> 's) -> 's
 
     val subst : t -> Term.t -> Term.t
     (** Apply a substitution recursively to subterms. *)
diff --git a/sledge/src/llair/exp.ml b/sledge/src/llair/exp.ml
index f6beddce0..9981a3672 100644
--- a/sledge/src/llair/exp.ml
+++ b/sledge/src/llair/exp.ml
@@ -410,24 +410,19 @@ let rec_record i typ = RecRecord (i, typ)
 
 (** Traverse *)
 
-let fold_exps e ~init ~f =
-  let rec fold_exps_ e z =
-    let z =
-      match e with
-      | Ap1 (_, _, x) -> fold_exps_ x z
-      | Ap2 (_, _, x, y) -> fold_exps_ y (fold_exps_ x z)
-      | Ap3 (_, _, w, x, y) -> fold_exps_ w (fold_exps_ y (fold_exps_ x z))
-      | ApN (_, _, xs) ->
-          IArray.fold xs ~init:z ~f:(fun z elt -> fold_exps_ elt z)
-      | _ -> z
-    in
-    f z e
-  in
-  fold_exps_ e init
-
-let fold_regs e ~init ~f =
-  fold_exps e ~init ~f:(fun z x ->
-      match x with Reg _ -> f z (x :> Reg.t) | _ -> z )
+let rec fold_exps e z ~f =
+  f e
+    ( match e with
+    | Ap1 (_, _, x) -> fold_exps ~f x z
+    | Ap2 (_, _, x, y) -> fold_exps ~f y (fold_exps ~f x z)
+    | Ap3 (_, _, w, x, y) ->
+        fold_exps ~f w (fold_exps ~f y (fold_exps ~f x z))
+    | ApN (_, _, xs) -> IArray.fold xs z ~f:(fold_exps ~f)
+    | _ -> z )
+
+let fold_regs e z ~f =
+  fold_exps e z ~f:(fun x z ->
+      match x with Reg _ -> f (x :> Reg.t) z | _ -> z )
 
 (** Query *)
 
diff --git a/sledge/src/llair/exp.mli b/sledge/src/llair/exp.mli
index e0c14e50b..761207a55 100644
--- a/sledge/src/llair/exp.mli
+++ b/sledge/src/llair/exp.mli
@@ -183,7 +183,7 @@ val rec_record : int -> Typ.t -> t
 
 (** Traverse *)
 
-val fold_regs : t -> init:'a -> f:('a -> Reg.t -> 'a) -> 'a
+val fold_regs : t -> 's -> f:(Reg.t -> 's -> 's) -> 's
 
 (** Query *)
 
diff --git a/sledge/src/llair/llair.ml b/sledge/src/llair/llair.ml
index 37013303f..2e0331b3b 100644
--- a/sledge/src/llair/llair.ml
+++ b/sledge/src/llair/llair.ml
@@ -260,9 +260,7 @@ module Inst = struct
   let union_locals inst vs =
     match inst with
     | Move {reg_exps; _} ->
-        IArray.fold
-          ~f:(fun vs (reg, _) -> Reg.Set.add reg vs)
-          ~init:vs reg_exps
+        IArray.fold ~f:(fun (reg, _) vs -> Reg.Set.add reg vs) reg_exps vs
     | Load {reg; _} | Alloc {reg; _} | Nondet {reg= Some reg; _} ->
         Reg.Set.add reg vs
     | Store _ | Memcpy _ | Memmov _ | Memset _ | Free _
@@ -272,19 +270,19 @@ module Inst = struct
 
   let locals inst = union_locals inst Reg.Set.empty
 
-  let fold_exps inst ~init ~f =
+  let fold_exps inst s ~f =
     match inst with
     | Move {reg_exps; loc= _} ->
-        IArray.fold reg_exps ~init ~f:(fun acc (_reg, exp) -> f acc exp)
-    | Load {reg= _; ptr; len; loc= _} -> f (f init ptr) len
-    | Store {ptr; exp; len; loc= _} -> f (f (f init ptr) exp) len
-    | Memset {dst; byt; len; loc= _} -> f (f (f init dst) byt) len
+        IArray.fold ~f:(fun (_reg, exp) -> f exp) reg_exps s
+    | Load {reg= _; ptr; len; loc= _} -> f len (f ptr s)
+    | Store {ptr; exp; len; loc= _} -> f len (f exp (f ptr s))
+    | Memset {dst; byt; len; loc= _} -> f len (f byt (f dst s))
     | Memcpy {dst; src; len; loc= _} | Memmov {dst; src; len; loc= _} ->
-        f (f (f init dst) src) len
-    | Alloc {reg= _; num; len= _; loc= _} -> f init num
-    | Free {ptr; loc= _} -> f init ptr
-    | Nondet {reg= _; msg= _; loc= _} -> init
-    | Abort {loc= _} -> init
+        f len (f src (f dst s))
+    | Alloc {reg= _; num; len= _; loc= _} -> f num s
+    | Free {ptr; loc= _} -> f ptr s
+    | Nondet {reg= _; msg= _; loc= _} -> s
+    | Abort {loc= _} -> s
 end
 
 (** Jumps *)
@@ -416,34 +414,27 @@ module Func = struct
     | {entry= {cmnd; term= Unreachable; _}; _} -> IArray.is_empty cmnd
     | _ -> false
 
-  let fold_cfg ~init ~f func =
+  let fold_cfg ~f func s =
     let seen = BlockS.create 0 in
-    let rec fold_cfg_ s blk =
+    let rec fold_cfg_ blk s =
       if not (BlockS.add seen blk) then s
       else
         let s =
-          let f s j = fold_cfg_ s j.dst in
+          let f j s = fold_cfg_ j.dst s in
           match blk.term with
           | Switch {tbl; els; _} ->
-              let s = IArray.fold ~f:(fun s (_, j) -> f s j) ~init:s tbl in
-              f s els
-          | Iswitch {tbl; _} -> IArray.fold ~f ~init:s tbl
-          | Call {return; throw; _} ->
-              let s = f s return in
-              Option.fold ~f ~init:s throw
+              let s = IArray.fold ~f:(fun (_, j) -> f j) tbl s in
+              f els s
+          | Iswitch {tbl; _} -> IArray.fold ~f tbl s
+          | Call {return; throw; _} -> Option.fold ~f throw (f return s)
           | Return _ | Throw _ | Unreachable -> s
         in
-        f s blk
+        f blk s
     in
-    fold_cfg_ init func.entry
+    fold_cfg_ func.entry s
 
-  let fold_term func ~init ~f =
-    fold_cfg func ~init ~f:(fun s blk -> f s blk.term)
-
-  let iter_term func ~f =
-    fold_cfg func ~init:() ~f:(fun () blk -> f blk.term)
-
-  let entry_cfg func = fold_cfg ~init:[] ~f:(fun cfg blk -> blk :: cfg) func
+  let iter_term func ~f = fold_cfg ~f:(fun blk () -> f blk.term) func ()
+  let entry_cfg func = fold_cfg ~f:(fun blk cfg -> blk :: cfg) func []
 
   let pp fs func =
     let {name; formals; freturn; entry; _} = func in
@@ -489,13 +480,12 @@ module Func = struct
   let mk ~(name : Global.t) ~formals ~freturn ~fthrow ~entry ~cfg =
     let locals =
       let locals_cmnd locals cmnd =
-        IArray.fold_right ~f:Inst.union_locals cmnd ~init:locals
+        IArray.fold_right ~f:Inst.union_locals cmnd locals
       in
-      let locals_block locals block =
+      let locals_block block locals =
         locals_cmnd (Term.union_locals block.term locals) block.cmnd
       in
-      let init = locals_block Reg.Set.empty entry in
-      IArray.fold ~f:locals_block cfg ~init
+      IArray.fold ~f:locals_block cfg (locals_block entry Reg.Set.empty)
     in
     let func = {name; formals; freturn; fthrow; locals; entry} in
     let resolve_parent_and_jumps block =
@@ -535,13 +525,14 @@ let set_derived_metadata functions =
     String.Map.iter functions ~f:(fun func ->
         FuncQ.enqueue_back_exn roots func.name.reg func ) ;
     String.Map.iter functions ~f:(fun func ->
-        Func.fold_term func ~init:() ~f:(fun () -> function
-          | Call {callee; _} -> (
-            match Reg.of_exp callee with
-            | Some callee ->
-                FuncQ.remove roots callee |> (ignore : [> ] -> unit)
-            | None -> () )
-          | _ -> () ) ) ;
+        Func.iter_term func ~f:(fun term ->
+            match term with
+            | Call {callee; _} -> (
+              match Reg.of_exp callee with
+              | Some callee ->
+                  FuncQ.remove roots callee |> (ignore : [> ] -> unit)
+              | None -> () )
+            | _ -> () ) ) ;
     roots
   in
   let topsort functions roots =
@@ -588,7 +579,7 @@ let set_derived_metadata functions =
         index := !index - 1 )
   in
   let functions =
-    List.fold functions ~init:String.Map.empty ~f:(fun m func ->
+    List.fold functions String.Map.empty ~f:(fun func m ->
         String.Map.add_exn ~key:(Reg.name func.name.reg) ~data:func m )
   in
   let roots = compute_roots functions in
diff --git a/sledge/src/llair/llair.mli b/sledge/src/llair/llair.mli
index 9e0b0d26b..dad96f0da 100644
--- a/sledge/src/llair/llair.mli
+++ b/sledge/src/llair/llair.mli
@@ -121,7 +121,7 @@ module Inst : sig
   val abort : loc:Loc.t -> inst
   val loc : inst -> Loc.t
   val locals : inst -> Reg.Set.t
-  val fold_exps : inst -> init:'a -> f:('a -> Exp.t -> 'a) -> 'a
+  val fold_exps : inst -> 's -> f:(Exp.t -> 's -> 's) -> 's
 end
 
 module Jump : sig
diff --git a/sledge/src/llair_to_Fol.ml b/sledge/src/llair_to_Fol.ml
index ae03337d8..2e1f5c654 100644
--- a/sledge/src/llair_to_Fol.ml
+++ b/sledge/src/llair_to_Fol.ml
@@ -17,9 +17,7 @@ let reg r =
   Var.program ~name ~global
 
 let regs rs =
-  Llair.Reg.Set.fold
-    ~f:(fun r -> Var.Set.add (reg r))
-    rs ~init:Var.Set.empty
+  Llair.Reg.Set.fold ~f:(fun r -> Var.Set.add (reg r)) rs Var.Set.empty
 
 let uap0 f = T.apply f [||]
 let uap1 f a = T.apply f [|a|]
diff --git a/sledge/src/ses/equality.ml b/sledge/src/ses/equality.ml
index f2257500b..98acc69ce 100644
--- a/sledge/src/ses/equality.ml
+++ b/sledge/src/ses/equality.ml
@@ -25,9 +25,9 @@ let interpreted e = equal_kind (classify e) Interpreted
 let non_interpreted e = not (interpreted e)
 let uninterpreted e = equal_kind (classify e) Uninterpreted
 
-let rec fold_max_solvables e ~init ~f =
-  if non_interpreted e then f e init
-  else Term.fold e ~init ~f:(fun d s -> fold_max_solvables ~f d ~init:s)
+let rec fold_max_solvables e s ~f =
+  if non_interpreted e then f e s
+  else Term.fold ~f:(fold_max_solvables ~f) e s
 
 (** Solution Substitutions *)
 module Subst : sig
@@ -40,7 +40,7 @@ module Subst : sig
   val length : t -> int
   val mem : Term.t -> t -> bool
   val find : Term.t -> t -> Term.t option
-  val fold : t -> init:'a -> f:(key:Term.t -> data:Term.t -> 'a -> 'a) -> 'a
+  val fold : t -> 's -> f:(key:Term.t -> data:Term.t -> 's -> 's) -> 's
   val iteri : t -> f:(key:Term.t -> data:Term.t -> unit) -> unit
   val for_alli : t -> f:(key:Term.t -> data:Term.t -> bool) -> bool
   val apply : t -> Term.t -> Term.t
@@ -116,13 +116,13 @@ end = struct
 
   (** remove entries for vars *)
   let remove xs s =
-    Var.Set.fold ~f:(fun x s -> Term.Map.remove (Term.var x) s) xs ~init:s
+    Var.Set.fold ~f:(fun x -> Term.Map.remove (Term.var x)) xs s
 
   (** map over a subst, applying [f] to both domain and range, requires that
       [f] is injective and for any set of terms [E], [f\[E\]] is disjoint
       from [E] *)
   let map_entries ~f s =
-    Term.Map.fold s ~init:s ~f:(fun ~key ~data s ->
+    Term.Map.fold s s ~f:(fun ~key ~data s ->
         let key' = f key in
         let data' = f data in
         if Term.equal key' key then
@@ -159,7 +159,7 @@ end = struct
        valid, so loop until no change. *)
     let rec partition_valid_ t ks s =
       let t', ks', s' =
-        Term.Map.fold s ~init:(t, ks, s) ~f:(fun ~key ~data (t, ks, s) ->
+        Term.Map.fold s (t, ks, s) ~f:(fun ~key ~data (t, ks, s) ->
             if is_valid_eq ks key data then (t, ks, s)
             else
               let t = Term.Map.add ~key ~data t
@@ -244,8 +244,8 @@ let rec solve_extract ?f a o l b s =
 (* α₀^…^αᵢ^αⱼ^…^αᵥ = β ==> |α₀^…^αᵥ| = |β| ∧ … ∧ αⱼ = β[n₀+…+nᵢ,nⱼ) ∧ …
    where nₓ ≡ |αₓ| and m = |β| *)
 and solve_concat ?f a0V b m s =
-  Iter.fold_until (IArray.to_iter a0V) ~init:(s, Term.zero)
-    ~f:(fun (s, oI) aJ ->
+  Iter.fold_until (IArray.to_iter a0V) (s, Term.zero)
+    ~f:(fun aJ (s, oI) ->
       let nJ = Term.seq_size_exn aJ in
       let oJ = Term.add oI nJ in
       match solve_ ?f aJ (Term.extract ~seq:b ~off:oI ~len:nJ) s with
@@ -344,7 +344,7 @@ let classes r =
     if Term.equal key data then cls
     else Term.Map.add_multi ~key:data ~data:key cls
   in
-  Subst.fold r.rep ~init:Term.Map.empty ~f:(fun ~key ~data cls ->
+  Subst.fold r.rep Term.Map.empty ~f:(fun ~key ~data cls ->
       match classify key with
       | Interpreted | Atomic -> add key data cls
       | Uninterpreted -> add (Term.map ~f:(Subst.apply r.rep) key) data cls )
@@ -477,12 +477,12 @@ let rec extend_ a r =
   match (a : Term.t) with
   | Integer _ | Rational _ -> r
   | _ -> (
-      if interpreted a then Term.fold ~f:extend_ a ~init:r
+      if interpreted a then Term.fold ~f:extend_ a r
       else
         (* add uninterpreted terms *)
         match Subst.extend a r with
         (* and their subterms if newly added *)
-        | Some r -> Term.fold ~f:extend_ a ~init:r
+        | Some r -> Term.fold ~f:extend_ a r
         | None -> r )
 
 (** add a term to the carrier *)
@@ -572,26 +572,22 @@ let class_of r e =
   let e' = normalize r e in
   e' :: Term.Map.find_multi e' (classes r)
 
-let fold_uses_of r t ~init ~f =
-  let rec fold_ e ~init:s ~f =
+let fold_uses_of r t s ~f =
+  let rec fold_ e s ~f =
     let s =
-      Term.fold e ~init:s ~f:(fun sub s ->
-          if Term.equal t sub then f s e else s )
+      Term.fold e s ~f:(fun sub s -> if Term.equal t sub then f s e else s)
     in
-    if interpreted e then
-      Term.fold e ~init:s ~f:(fun d s -> fold_ ~f d ~init:s)
-    else s
+    if interpreted e then Term.fold ~f:(fold_ ~f) e s else s
   in
-  Subst.fold r.rep ~init ~f:(fun ~key:trm ~data:rep s ->
-      let f trm s = fold_ trm ~init:s ~f in
-      f trm (f rep s) )
+  Subst.fold r.rep s ~f:(fun ~key:trm ~data:rep s ->
+      fold_ ~f trm (fold_ ~f rep s) )
 
 let apply_subst us s r =
   [%Trace.call fun {pf} -> pf "%a@ %a" Subst.pp s pp r]
   ;
-  Term.Map.fold (classes r) ~init:true_ ~f:(fun ~key:rep ~data:cls r ->
+  Term.Map.fold (classes r) true_ ~f:(fun ~key:rep ~data:cls r ->
       let rep' = Subst.subst s rep in
-      List.fold cls ~init:r ~f:(fun r trm ->
+      List.fold cls r ~f:(fun trm r ->
           let trm' = Subst.subst s trm in
           and_eq_ us trm' rep' r ) )
   |> extract_xs
@@ -609,8 +605,7 @@ let and_ us r s =
     let s, r =
       if Subst.length s.rep <= Subst.length r.rep then (s, r) else (r, s)
     in
-    Subst.fold s.rep ~init:r ~f:(fun ~key:e ~data:e' r -> and_eq_ us e e' r)
-  )
+    Subst.fold s.rep r ~f:(fun ~key:e ~data:e' r -> and_eq_ us e e' r) )
   |> extract_xs
   |>
   [%Trace.retn fun {pf} (_, r') ->
@@ -624,10 +619,10 @@ let or_ us r s =
   else if not r.sat then s
   else
     let merge_mems rs r s =
-      Term.Map.fold (classes s) ~init:rs ~f:(fun ~key:rep ~data:cls rs ->
+      Term.Map.fold (classes s) rs ~f:(fun ~key:rep ~data:cls rs ->
           List.fold cls
-            ~init:([rep], rs)
-            ~f:(fun (reps, rs) exp ->
+            ([rep], rs)
+            ~f:(fun exp (reps, rs) ->
               match
                 List.find ~f:(fun rep -> implies r (Term.eq exp rep)) reps
               with
@@ -648,13 +643,13 @@ let or_ us r s =
 let orN us rs =
   match rs with
   | [] -> (us, false_)
-  | r :: rs -> List.fold ~f:(fun (us, s) r -> or_ us s r) ~init:(us, r) rs
+  | r :: rs -> List.fold ~f:(fun r (us, s) -> or_ us s r) rs (us, r)
 
 let rec and_term_ us e r =
   let eq_false b r = and_eq_ us b Term.false_ r in
   match (e : Term.t) with
   | Integer {data} -> if Z.is_false data then false_ else r
-  | And cs -> Term.Set.fold ~f:(and_term_ us) cs ~init:r
+  | And cs -> Term.Set.fold ~f:(and_term_ us) cs r
   | Ap2 (Eq, a, b) -> and_eq_ us a b r
   | Ap2 (Xor, Integer {data}, a) when Z.is_true data -> eq_false a r
   | Ap2 (Xor, a, Integer {data}) when Z.is_true data -> eq_false a r
@@ -688,11 +683,10 @@ let rename r sub =
     pf "%a" pp_diff (r, r') ;
     invariant r']
 
-let fold_terms r ~init ~f =
-  Subst.fold r.rep ~f:(fun ~key ~data z -> f (f z data) key) ~init
+let fold_terms r z ~f =
+  Subst.fold ~f:(fun ~key ~data z -> f key (f data z)) r.rep z
 
-let fold_vars r ~init ~f =
-  fold_terms r ~init ~f:(fun init -> Term.fold_vars ~f ~init)
+let fold_vars r z ~f = fold_terms ~f:(Term.fold_vars ~f) r z
 
 (** Existential Witnessing and Elimination *)
 
@@ -721,7 +715,7 @@ let solve_poly_eq us p' q' subst =
   ;
   let diff = Term.sub p' q' in
   let max_solvables_not_ito_us =
-    fold_max_solvables diff ~init:Zero ~f:(fun solvable_subterm -> function
+    fold_max_solvables diff Zero ~f:(fun solvable_subterm -> function
       | Many -> Many
       | zom when Var.Set.subset (Term.fv solvable_subterm) ~of_:us -> zom
       | One _ -> Many
@@ -833,8 +827,8 @@ let solve_uninterp_eqs us (cls, subst) =
     [%compare: kind * Term.t] (classify e, e) (classify f, f)
   in
   let {rep_us; cls_us; rep_xs; cls_xs} =
-    List.fold cls ~init:{rep_us= None; cls_us= []; rep_xs= None; cls_xs= []}
-      ~f:(fun ({rep_us; cls_us; rep_xs; cls_xs} as s) trm ->
+    List.fold cls {rep_us= None; cls_us= []; rep_xs= None; cls_xs= []}
+      ~f:(fun trm ({rep_us; cls_us; rep_xs; cls_xs} as s) ->
         if Var.Set.subset (Term.fv trm) ~of_:us then
           match rep_us with
           | Some rep when compare rep trm <= 0 ->
@@ -867,7 +861,7 @@ let solve_uninterp_eqs us (cls, subst) =
         | None -> (cls, cls_xs)
       in
       let subst =
-        List.fold cls_xs ~init:subst ~f:(fun subst trm_xs ->
+        List.fold cls_xs subst ~f:(fun trm_xs subst ->
             Subst.compose1 ~key:trm_xs ~data:rep_us subst )
       in
       (cls, subst)
@@ -876,7 +870,7 @@ let solve_uninterp_eqs us (cls, subst) =
     | Some rep_xs ->
         let cls = rep_xs :: cls_us in
         let subst =
-          List.fold cls_xs ~init:subst ~f:(fun subst trm_xs ->
+          List.fold cls_xs subst ~f:(fun trm_xs subst ->
               Subst.compose1 ~key:trm_xs ~data:rep_xs subst )
         in
         (cls, subst)
@@ -920,9 +914,9 @@ let solve_concat_extracts_eq r x =
   [%Trace.call fun {pf} -> pf "%a@ %a" Term.pp x pp r]
   ;
   let uses =
-    fold_uses_of r x ~init:[] ~f:(fun uses -> function
+    fold_uses_of r x [] ~f:(fun uses -> function
       | Ap2 (Sized, _, _) as m ->
-          fold_uses_of r m ~init:uses ~f:(fun uses -> function
+          fold_uses_of r m uses ~f:(fun uses -> function
             | Ap3 (Extract, _, _, _) as e -> e :: uses | _ -> uses )
       | _ -> uses )
   in
@@ -933,8 +927,8 @@ let solve_concat_extracts_eq r x =
         | _ -> false )
   in
   let rec find_extracts full_rev_extracts rev_prefix off =
-    List.fold (find_extracts_at_off off) ~init:full_rev_extracts
-      ~f:(fun full_rev_extracts e ->
+    List.fold (find_extracts_at_off off) full_rev_extracts
+      ~f:(fun e full_rev_extracts ->
         match e with
         | Ap3 (Extract, Ap2 (Sized, n, _), o, l) ->
             let o_l = Term.add o l in
@@ -951,10 +945,9 @@ let solve_concat_extracts_eq r x =
 let solve_concat_extracts r us x (classes, subst, us_xs) =
   match
     List.filter_map (solve_concat_extracts_eq r x) ~f:(fun rev_extracts ->
-        Iter.fold_opt (Iter.of_list rev_extracts) ~init:[]
-          ~f:(fun suffix e ->
+        Iter.fold_opt (Iter.of_list rev_extracts) [] ~f:(fun e suffix ->
             let+ rep_ito_us =
-              List.fold (cls_of r e) ~init:None ~f:(fun rep_ito_us trm ->
+              List.fold (cls_of r e) None ~f:(fun trm rep_ito_us ->
                   match rep_ito_us with
                   | Some rep when Term.compare rep trm <= 0 -> rep_ito_us
                   | _ when Var.Set.subset (Term.fv trm) ~of_:us -> Some trm
@@ -970,9 +963,8 @@ let solve_concat_extracts r us x (classes, subst, us_xs) =
       (classes, subst, us_xs)
   | None -> (classes, subst, us_xs)
 
-let solve_for_xs r us xs (classes, subst, us_xs) =
-  Var.Set.fold xs ~init:(classes, subst, us_xs)
-    ~f:(fun x (classes, subst, us_xs) ->
+let solve_for_xs r us xs =
+  Var.Set.fold xs ~f:(fun x (classes, subst, us_xs) ->
       let x = Term.var x in
       if Subst.mem x subst then (classes, subst, us_xs)
       else solve_concat_extracts r us x (classes, subst, us_xs) )
@@ -980,14 +972,13 @@ let solve_for_xs r us xs (classes, subst, us_xs) =
 (** move equations from [classes] to [subst] which can be expressed, after
     normalizing with [subst], as [x ↦ u] where [us ∪ xs ⊇ fv x ⊈ us]
     and [fv u ⊆ us] or else [fv u ⊆ us ∪ xs]. *)
-let solve_classes r (classes, subst, us) xs =
+let solve_classes r xs (classes, subst, us) =
   [%Trace.call fun {pf} ->
     pf "us: {@[%a@]}@ xs: {@[%a@]}" Var.Set.pp us Var.Set.pp xs]
   ;
   let rec solve_classes_ (classes0, subst0, us_xs) =
     let classes, subst =
-      Term.Map.fold ~f:(solve_class us us_xs) classes0
-        ~init:(classes0, subst0)
+      Term.Map.fold ~f:(solve_class us us_xs) classes0 (classes0, subst0)
     in
     if subst != subst0 then solve_classes_ (classes, subst, us_xs)
     else (classes, subst, us_xs)
@@ -1018,8 +1009,7 @@ let solve_for_vars vss r =
   let us, vss =
     match vss with us :: vss -> (us, vss) | [] -> (Var.Set.empty, vss)
   in
-  List.fold ~f:(solve_classes r) ~init:(classes r, Subst.empty, us) vss
-  |> snd3
+  List.fold ~f:(solve_classes r) vss (classes r, Subst.empty, us) |> snd3
   |>
   [%Trace.retn fun {pf} subst ->
     pf "%a" Subst.pp subst ;
@@ -1029,8 +1019,8 @@ let solve_for_vars vss r =
           || fail "@[%a@ = %a@ not entailed by@ @[%a@]@]" Term.pp key
                Term.pp data pp_classes r () ) ;
         assert (
-          Iter.fold_until (Iter.of_list vss) ~init:us
-            ~f:(fun us xs ->
+          Iter.fold_until (Iter.of_list vss) us
+            ~f:(fun xs us ->
               let us_xs = Var.Set.union us xs in
               let ks = Term.fv key in
               let ds = Term.fv data in
diff --git a/sledge/src/ses/equality.mli b/sledge/src/ses/equality.mli
index e78b955cb..6e826038a 100644
--- a/sledge/src/ses/equality.mli
+++ b/sledge/src/ses/equality.mli
@@ -65,7 +65,7 @@ val normalize : t -> Term.t -> Term.t
     relation, where [e'] and its subterms are expressed in terms of the
     relation's canonical representatives of each equivalence class. *)
 
-val fold_vars : t -> init:'a -> f:('a -> Var.t -> 'a) -> 'a
+val fold_vars : t -> 's -> f:(Var.t -> 's -> 's) -> 's
 
 (** Solution Substitutions *)
 module Subst : sig
@@ -73,7 +73,7 @@ module Subst : sig
 
   val pp : t pp
   val is_empty : t -> bool
-  val fold : t -> init:'a -> f:(key:Term.t -> data:Term.t -> 'a -> 'a) -> 'a
+  val fold : t -> 's -> f:(key:Term.t -> data:Term.t -> 's -> 's) -> 's
 
   val subst : t -> Term.t -> Term.t
   (** Apply a substitution recursively to subterms. *)
diff --git a/sledge/src/ses/term.ml b/sledge/src/ses/term.ml
index acc3c1c42..6b03b7d1e 100644
--- a/sledge/src/ses/term.ml
+++ b/sledge/src/ses/term.ml
@@ -373,9 +373,7 @@ module Sum = struct
     | _ -> Qset.add term coeff sum
 
   let of_ ?(coeff = Q.one) term = add coeff term empty
-
-  let map sum ~f =
-    Qset.fold sum ~init:empty ~f:(fun e c sum -> add c (f e) sum)
+  let map sum ~f = Qset.fold ~f:(fun e c sum -> add c (f e) sum) sum empty
 
   let mul_const const sum =
     assert (not (Q.equal Q.zero const)) ;
@@ -436,7 +434,7 @@ let rec simp_add_ es poly =
     (* (c₁ × X₁) + X₂ ==> ∑ᵢ₌₁² cᵢ × Xᵢ for c₂ = 1 *)
     | _ -> Sum.to_term (Sum.add coeff term (Sum.of_ poly))
   in
-  Qset.fold ~f es ~init:poly
+  Qset.fold ~f es poly
 
 and simp_mul2 e f =
   match (e, f) with
@@ -526,7 +524,7 @@ let simp_mul es =
     if Q.equal Q.zero pwr then term
     else mul_pwr bas Q.(pwr - one) (simp_mul2 bas term)
   in
-  Qset.fold es ~init:one ~f:(fun bas pwr term ->
+  Qset.fold es one ~f:(fun bas pwr term ->
       if Q.sign pwr >= 0 then mul_pwr bas pwr term
       else simp_div term (mul_pwr bas (Q.neg pwr) one) )
 
@@ -569,7 +567,7 @@ let rec simp_and2 x y =
       let add s = function And cs -> Set.union s cs | c -> Set.add c s in
       And (add (add Set.empty x) y)
 
-let simp_and xs = Set.fold xs ~init:true_ ~f:simp_and2
+let simp_and xs = Set.fold ~f:simp_and2 xs true_
 
 let rec simp_or2 x y =
   match (x, y) with
@@ -590,15 +588,16 @@ let rec simp_or2 x y =
       let add s = function Or cs -> Set.union s cs | c -> Set.add c s in
       Or (add (add Set.empty x) y)
 
-let simp_or xs = Set.fold xs ~init:false_ ~f:simp_or2
+let simp_or xs = Set.fold ~f:simp_or2 xs false_
 
 (* sequence sizes *)
 
 let rec seq_size_exn = function
   | Ap2 (Sized, n, _) | Ap3 (Extract, _, _, n) -> n
   | ApN (Concat, a0U) ->
-      IArray.fold a0U ~init:zero ~f:(fun a0I aJ ->
-          simp_add2 a0I (seq_size_exn aJ) )
+      IArray.fold
+        ~f:(fun aJ a0I -> simp_add2 a0I (seq_size_exn aJ))
+        a0U zero
   | _ -> invalid_arg "seq_size_exn"
 
 let seq_size e = try Some (seq_size_exn e) with Invalid_argument _ -> None
@@ -666,11 +665,11 @@ let rec simp_extract seq off len =
     | ApN (Concat, na1N) -> (
       match len with
       | Integer {data= l} ->
-          IArray.fold_map_until na1N ~init:(l, off)
-            ~f:(fun (l, oI) naI ->
+          IArray.fold_map_until na1N (l, off)
+            ~f:(fun naI (l, oI) ->
               let nI = seq_size_exn naI in
               if Z.equal Z.zero l then
-                `Continue ((l, oI), simp_extract naI oI zero)
+                `Continue (simp_extract naI oI zero, (l, oI))
               else
                 let oI_nI = simp_sub oI nI in
                 match oI_nI with
@@ -678,9 +677,9 @@ let rec simp_extract seq off len =
                     let oJ = if Z.sign data <= 0 then zero else oI_nI in
                     let lI = Z.(max zero (min l (neg data))) in
                     let l = Z.(l - lI) in
-                    `Continue ((l, oJ), simp_extract naI oI (integer lI))
+                    `Continue (simp_extract naI oI (integer lI), (l, oJ))
                 | _ -> `Stop (Ap3 (Extract, seq, off, len)) )
-            ~finish:(fun (_, e1N) -> simp_concat e1N)
+            ~finish:(fun (e1N, _) -> simp_concat e1N)
       | _ -> Ap3 (Extract, seq, off, len) )
     (* α[o,l) *)
     | _ -> Ap3 (Extract, seq, off, len) )
@@ -697,7 +696,7 @@ and simp_concat xs =
     in
     let concat_sub_Concat xs =
       IArray.concat
-        (IArray.fold_right xs ~init:[] ~f:(fun x s ->
+        (IArray.fold_right xs [] ~f:(fun x s ->
              match x with
              | ApN (Concat, ys) -> ys :: s
              | x -> IArray.of_array [|x|] :: s ))
@@ -1018,31 +1017,30 @@ let map e ~f =
   | NegLit (sym, xs) -> mapN (simp_neglit sym) ~f xs
   | Var _ | Integer _ | Rational _ | RecRecord _ -> e
 
-let fold_map e ~init ~f =
-  let s = ref init in
+let fold_map e s0 ~f =
+  let s = ref s0 in
   let f x =
-    let s', x' = f !s x in
+    let x', s' = f x !s in
     s := s' ;
     x'
   in
   let e' = map e ~f in
-  (!s, e')
+  (e', !s)
 
 let rec map_rec_pre e ~f =
   match f e with Some e' -> e' | None -> map ~f:(map_rec_pre ~f) e
 
-let rec fold_map_rec_pre e ~init:s ~f =
-  match f s e with
-  | Some (s, e') -> (s, e')
-  | None -> fold_map ~f:(fun s e -> fold_map_rec_pre ~f ~init:s e) ~init:s e
+let rec fold_map_rec_pre e s ~f =
+  match f e s with
+  | Some (e', s) -> (e', s)
+  | None -> fold_map ~f:(fold_map_rec_pre ~f) e s
 
 let disjuncts e =
   let rec disjuncts_ e =
     match e with
     | Or es ->
         let e0, e1N = Set.pop_exn es in
-        Set.fold e1N ~init:(disjuncts_ e0) ~f:(fun e cs ->
-            Set.union cs (disjuncts_ e) )
+        Set.fold ~f:(fun e -> Set.union (disjuncts_ e)) e1N (disjuncts_ e0)
     | Ap3 (Conditional, cnd, thn, els) ->
         Set.add
           (and_ (orN (disjuncts_ (not_ cnd))) (orN (disjuncts_ els)))
@@ -1095,15 +1093,15 @@ let for_all e ~f =
   | Add args | Mul args -> Qset.for_all ~f:(fun arg _ -> f arg) args
   | Var _ | Integer _ | Rational _ | RecRecord _ -> true
 
-let fold e ~init:s ~f =
+let fold e s ~f =
   match e with
   | Ap1 (_, x) -> f x s
   | Ap2 (_, x, y) -> f y (f x s)
   | Ap3 (_, x, y, z) -> f z (f y (f x s))
   | ApN (_, xs) | Apply (_, xs) | PosLit (_, xs) | NegLit (_, xs) ->
-      IArray.fold ~f:(fun s x -> f x s) xs ~init:s
-  | And args | Or args -> Set.fold ~f args ~init:s
-  | Add args | Mul args -> Qset.fold ~f:(fun e _ s -> f e s) args ~init:s
+      IArray.fold ~f xs s
+  | And args | Or args -> Set.fold ~f args s
+  | Add args | Mul args -> Qset.fold ~f:(fun e _ s -> f e s) args s
   | Var _ | Integer _ | Rational _ | RecRecord _ -> s
 
 let rec iter_terms e ~f =
@@ -1124,21 +1122,19 @@ let rec iter_terms e ~f =
   | Var _ | Integer _ | Rational _ | RecRecord _ -> () ) ;
   f e
 
-let rec fold_terms e ~init:s ~f =
-  let fold_terms f e s = fold_terms e ~init:s ~f in
-  let s =
-    match e with
-    | Ap1 (_, x) -> fold_terms f x s
-    | Ap2 (_, x, y) -> fold_terms f y (fold_terms f x s)
-    | Ap3 (_, x, y, z) -> fold_terms f z (fold_terms f y (fold_terms f x s))
+let rec fold_terms e s ~f =
+  f e
+    ( match e with
+    | Ap1 (_, x) -> fold_terms ~f x s
+    | Ap2 (_, x, y) -> fold_terms ~f y (fold_terms ~f x s)
+    | Ap3 (_, x, y, z) ->
+        fold_terms ~f z (fold_terms ~f y (fold_terms ~f x s))
     | ApN (_, xs) | Apply (_, xs) | PosLit (_, xs) | NegLit (_, xs) ->
-        IArray.fold ~f:(fun s x -> fold_terms f x s) xs ~init:s
-    | And args | Or args -> Set.fold args ~init:s ~f:(fold_terms f)
+        IArray.fold ~f:(fold_terms ~f) xs s
+    | And args | Or args -> Set.fold ~f:(fold_terms ~f) args s
     | Add args | Mul args ->
-        Qset.fold args ~init:s ~f:(fun arg _ s -> fold_terms f arg s)
-    | Var _ | Integer _ | Rational _ | RecRecord _ -> s
-  in
-  f s e
+        Qset.fold ~f:(fun arg _ -> fold_terms ~f arg) args s
+    | Var _ | Integer _ | Rational _ | RecRecord _ -> s )
 
 let iter_vars e ~f =
   iter_terms ~f:(fun e -> Option.iter ~f (Var.of_term e)) e
@@ -1146,12 +1142,12 @@ let iter_vars e ~f =
 let exists_vars e ~f =
   Iter.exists ~f (Iter.from_labelled_iter (iter_vars e))
 
-let fold_vars e ~init ~f =
-  fold_terms ~f:(fun s e -> Option.fold ~f ~init:s (Var.of_term e)) ~init e
+let fold_vars e s ~f =
+  fold_terms ~f:(fun e -> Option.fold ~f (Var.of_term e)) e s
 
 (** Query *)
 
-let fv e = fold_vars e ~f:(Fun.flip Var.Set.add) ~init:Var.Set.empty
+let fv e = fold_vars ~f:Var.Set.add e Var.Set.empty
 let is_true = function Integer {data} -> Z.is_true data | _ -> false
 let is_false = function Integer {data} -> Z.is_false data | _ -> false
 
@@ -1166,11 +1162,9 @@ let rec height = function
   | Ap2 (_, a, b) -> 1 + max (height a) (height b)
   | Ap3 (_, a, b, c) -> 1 + max (height a) (max (height b) (height c))
   | ApN (_, v) | Apply (_, v) | PosLit (_, v) | NegLit (_, v) ->
-      1 + IArray.fold v ~init:0 ~f:(fun m a -> max m (height a))
-  | And bs | Or bs ->
-      1 + Set.fold bs ~init:0 ~f:(fun a m -> max m (height a))
-  | Add qs | Mul qs ->
-      1 + Qset.fold qs ~init:0 ~f:(fun a _ m -> max m (height a))
+      1 + IArray.fold ~f:(fun a m -> max m (height a)) v 0
+  | And bs | Or bs -> 1 + Set.fold ~f:(fun a m -> max m (height a)) bs 0
+  | Add qs | Mul qs -> 1 + Qset.fold ~f:(fun a _ m -> max m (height a)) qs 0
   | Integer _ | Rational _ | RecRecord _ -> 0
 
 (** Solve *)
diff --git a/sledge/src/ses/term.mli b/sledge/src/ses/term.mli
index 2f9c2d7a7..6cc295367 100644
--- a/sledge/src/ses/term.mli
+++ b/sledge/src/ses/term.mli
@@ -199,11 +199,8 @@ val map_rec_pre : t -> f:(t -> t option) -> t
     to the subterms of [x], followed by rebuilding the term structure on the
     transformed subterms. *)
 
-val fold_map : t -> init:'a -> f:('a -> t -> 'a * t) -> 'a * t
-
-val fold_map_rec_pre :
-  t -> init:'a -> f:('a -> t -> ('a * t) option) -> 'a * t
-
+val fold_map : t -> 's -> f:(t -> 's -> t * 's) -> t * 's
+val fold_map_rec_pre : t -> 's -> f:(t -> 's -> (t * 's) option) -> t * 's
 val disjuncts : t -> t list
 val rename : (Var.t -> Var.t) -> t -> t
 
@@ -211,9 +208,9 @@ val rename : (Var.t -> Var.t) -> t -> t
 
 val iter : t -> f:(t -> unit) -> unit
 val exists : t -> f:(t -> bool) -> bool
-val fold : t -> init:'a -> f:(t -> 'a -> 'a) -> 'a
-val fold_vars : t -> init:'a -> f:('a -> Var.t -> 'a) -> 'a
-val fold_terms : t -> init:'a -> f:('a -> t -> 'a) -> 'a
+val fold : t -> 'a -> f:(t -> 'a -> 'a) -> 'a
+val fold_vars : t -> 'a -> f:(Var.t -> 'a -> 'a) -> 'a
+val fold_terms : t -> 'a -> f:(t -> 'a -> 'a) -> 'a
 
 (** Query *)
 
diff --git a/sledge/src/ses/var0.ml b/sledge/src/ses/var0.ml
index 3c6c03577..7964d6a81 100644
--- a/sledge/src/ses/var0.ml
+++ b/sledge/src/ses/var0.ml
@@ -66,7 +66,7 @@ module Make (T : REPR) = struct
     let invariant s =
       let@ () = Invariant.invariant [%here] s [%sexp_of: t] in
       let domain, range =
-        Map.fold s ~init:(Set.empty, Set.empty)
+        Map.fold s (Set.empty, Set.empty)
           ~f:(fun ~key ~data (domain, range) ->
             (* substs are injective *)
             assert (not (Set.mem range data)) ;
@@ -84,8 +84,7 @@ module Make (T : REPR) = struct
       else
         let wrt = Set.union wrt vs in
         let sub, rng, wrt =
-          Set.fold dom ~init:(empty, Set.empty, wrt)
-            ~f:(fun x (sub, rng, wrt) ->
+          Set.fold dom (empty, Set.empty, wrt) ~f:(fun x (sub, rng, wrt) ->
               let x', wrt = fresh (name x) ~wrt in
               let sub = Map.add_exn ~key:x ~data:x' sub in
               let rng = Set.add x' rng in
@@ -94,24 +93,21 @@ module Make (T : REPR) = struct
         ({sub; dom; rng}, wrt) )
       |> check (fun ({sub; _}, _) -> invariant sub)
 
-    let fold sub ~init ~f =
-      Map.fold sub ~init ~f:(fun ~key ~data s -> f key data s)
+    let fold sub z ~f = Map.fold ~f:(fun ~key ~data -> f key data) sub z
 
     let domain sub =
-      Map.fold sub ~init:Set.empty ~f:(fun ~key ~data:_ domain ->
-          Set.add key domain )
+      Map.fold ~f:(fun ~key ~data:_ -> Set.add key) sub Set.empty
 
     let range sub =
-      Map.fold sub ~init:Set.empty ~f:(fun ~key:_ ~data range ->
-          Set.add data range )
+      Map.fold ~f:(fun ~key:_ ~data -> Set.add data) sub Set.empty
 
     let invert sub =
-      Map.fold sub ~init:empty ~f:(fun ~key ~data sub' ->
+      Map.fold sub empty ~f:(fun ~key ~data sub' ->
           Map.add_exn ~key:data ~data:key sub' )
       |> check invariant
 
     let restrict sub vs =
-      Map.fold sub ~init:{sub; dom= Set.empty; rng= Set.empty}
+      Map.fold sub {sub; dom= Set.empty; rng= Set.empty}
         ~f:(fun ~key ~data z ->
           if Set.mem vs key then
             {z with dom= Set.add key z.dom; rng= Set.add data z.rng}
diff --git a/sledge/src/ses/var_intf.ml b/sledge/src/ses/var_intf.ml
index 7ddb070ff..3c5fbe974 100644
--- a/sledge/src/ses/var_intf.ml
+++ b/sledge/src/ses/var_intf.ml
@@ -64,7 +64,7 @@ module type VAR = sig
     val is_empty : t -> bool
     val domain : t -> Set.t
     val range : t -> Set.t
-    val fold : t -> init:'a -> f:(var -> var -> 'a -> 'a) -> 'a
+    val fold : t -> 's -> f:(var -> var -> 's -> 's) -> 's
     val apply : t -> var -> var
   end
 end
diff --git a/sledge/src/sh.ml b/sledge/src/sh.ml
index 42fb1cb04..f74423992 100644
--- a/sledge/src/sh.ml
+++ b/sledge/src/sh.ml
@@ -68,24 +68,21 @@ let map ~f_sjn ~f_ctx ~f_trm ~f_fml ({us; xs= _; ctx; pure; heap; djns} as q)
     then q
     else {q with ctx; pure; heap; djns}
 
-let fold_terms_seg {loc; bas; len; siz; seq} ~init ~f =
-  let f b s = f s b in
-  f loc (f bas (f len (f siz (f seq init))))
+let fold_terms_seg {loc; bas; len; siz; seq} s ~f =
+  f loc (f bas (f len (f siz (f seq s))))
 
-let fold_vars_seg seg ~init ~f =
-  fold_terms_seg seg ~init ~f:(fun init -> Term.fold_vars ~f ~init)
+let fold_vars_seg seg s ~f = fold_terms_seg ~f:(Term.fold_vars ~f) seg s
 
 let fold_vars_stem ?ignore_ctx ?ignore_pure
-    {us= _; xs= _; ctx; pure; heap; djns= _} ~init ~f =
-  let unless flag f init = if Option.is_some flag then init else f ~init in
-  List.fold ~f:(fun init -> fold_vars_seg ~f ~init) heap ~init
+    {us= _; xs= _; ctx; pure; heap; djns= _} s ~f =
+  let unless flag f s = if Option.is_some flag then s else f s in
+  List.fold ~f:(fold_vars_seg ~f) heap s
   |> unless ignore_pure (Formula.fold_vars ~f pure)
   |> unless ignore_ctx (Context.fold_vars ~f ctx)
 
-let fold_vars ?ignore_ctx ?ignore_pure fold_vars q ~init ~f =
-  fold_vars_stem ?ignore_ctx ?ignore_pure ~init ~f q
-  |> fun init ->
-  List.fold ~init q.djns ~f:(fun init -> List.fold ~init ~f:fold_vars)
+let fold_vars ?ignore_ctx ?ignore_pure fold_vars q s ~f =
+  fold_vars_stem ?ignore_ctx ?ignore_pure ~f q s
+  |> List.fold ~f:(List.fold ~f:fold_vars) q.djns
 
 (** Pretty-printing *)
 
@@ -98,13 +95,13 @@ let rec var_strength_ xs m q =
   in
   let xs = Var.Set.union xs q.xs in
   let m_stem =
-    fold_vars_stem ~ignore_ctx:() q ~init:m ~f:(fun m var ->
+    fold_vars_stem ~ignore_ctx:() q m ~f:(fun var m ->
         if not (Var.Set.mem xs var) then
           Var.Map.add ~key:var ~data:`Universal m
         else add var m )
   in
   let m =
-    List.fold ~init:m_stem q.djns ~f:(fun m djn ->
+    List.fold q.djns m_stem ~f:(fun djn m ->
         let ms = List.map ~f:(fun dj -> snd (var_strength_ xs m dj)) djn in
         List.reduce ms ~f:(fun m1 m2 ->
             Var.Map.union m1 m2 ~f:(fun _ s1 s2 ->
@@ -118,8 +115,8 @@ let rec var_strength_ xs m q =
 
 let var_strength ?(xs = Var.Set.empty) q =
   let m =
-    Var.Set.fold xs ~init:Var.Map.empty ~f:(fun x m ->
-        Var.Map.add ~key:x ~data:`Existential m )
+    Var.Set.fold xs Var.Map.empty ~f:(fun x ->
+        Var.Map.add ~key:x ~data:`Existential )
   in
   var_strength_ xs m q
 
@@ -146,7 +143,7 @@ let pp_block x fs segs =
         match Term.d_int len with
         | Some data -> (
           match
-            List.fold segs ~init:(Some Z.zero) ~f:(fun len seg ->
+            List.fold segs (Some Z.zero) ~f:(fun seg len ->
                 match (len, Term.d_int seg.siz) with
                 | Some len, Some data -> Some (Z.add len data)
                 | _ -> None )
@@ -258,17 +255,15 @@ let pp_djn fs d =
 let pp_raw fs q =
   pp_ ?var_strength:None Var.Set.empty Var.Set.empty Context.empty fs q
 
-let fv_seg seg =
-  fold_vars_seg seg ~f:(Fun.flip Var.Set.add) ~init:Var.Set.empty
+let fv_seg seg = fold_vars_seg ~f:Var.Set.add seg Var.Set.empty
 
 let fv ?ignore_ctx ?ignore_pure q =
-  let rec fv_union init q =
+  let rec fv_union q s =
     Var.Set.diff
-      (fold_vars ?ignore_ctx ?ignore_pure fv_union q ~init
-         ~f:(Fun.flip Var.Set.add))
+      (fold_vars ?ignore_ctx ?ignore_pure fv_union ~f:Var.Set.add q s)
       q.xs
   in
-  fv_union Var.Set.empty q
+  fv_union q Var.Set.empty
 
 let invariant_pure p = assert (not Formula.(equal ff p))
 let invariant_seg _ = ()
@@ -482,7 +477,7 @@ let star q1 q2 =
 let starN = function
   | [] -> emp
   | [q] -> q
-  | q :: qs -> List.fold ~f:star ~init:q qs
+  | q :: qs -> List.fold ~f:star qs q
 
 let or_ q1 q2 =
   [%Trace.call fun {pf} -> pf "(%a)@ (%a)" pp_raw q1 pp_raw q2]
@@ -514,13 +509,13 @@ let or_ q1 q2 =
 let orN = function
   | [] -> false_ Var.Set.empty
   | [q] -> q
-  | q :: qs -> List.fold ~f:or_ ~init:q qs
+  | q :: qs -> List.fold ~f:or_ qs q
 
 let pure (p : Formula.t) =
   [%Trace.call fun {pf} -> pf "%a" Formula.pp p]
   ;
-  Iter.fold (Context.dnf p) ~init:(false_ Var.Set.empty)
-    ~f:(fun q (xs, pure, ctx) ->
+  Iter.fold (Context.dnf p) (false_ Var.Set.empty)
+    ~f:(fun (xs, pure, ctx) q ->
       let us = Formula.fv pure in
       if Context.is_unsat ctx then extend_us us q
       else or_ q (exists_fresh xs {emp with us; ctx; pure}) )
@@ -536,7 +531,7 @@ let and_subst subst q =
   ;
   Context.Subst.fold
     ~f:(fun ~key ~data -> and_ (Formula.eq key data))
-    subst ~init:q
+    subst q
   |>
   [%Trace.retn fun {pf} q ->
     pf "%a" pp q ;
@@ -546,7 +541,7 @@ let subst sub q =
   [%Trace.call fun {pf} -> pf "@[%a@]@ %a" Var.Subst.pp sub pp q]
   ;
   let dom, eqs =
-    Var.Subst.fold sub ~init:(Var.Set.empty, Formula.tt)
+    Var.Subst.fold sub (Var.Set.empty, Formula.tt)
       ~f:(fun var trm (dom, eqs) ->
         ( Var.Set.add var dom
         , Formula.and_ (Formula.eq (Term.var var) (Term.var trm)) eqs ) )
@@ -580,11 +575,9 @@ let rec is_empty q =
 let rec pure_approx q =
   Formula.andN
     ( [q.pure]
-    |> fun init ->
-    List.fold ~init q.heap ~f:(fun p seg -> Formula.dq0 seg.loc :: p)
-    |> fun init ->
-    List.fold ~init q.djns ~f:(fun p djn ->
-        Formula.orN (List.map djn ~f:pure_approx) :: p ) )
+    |> List.fold q.heap ~f:(fun seg p -> Formula.dq0 seg.loc :: p)
+    |> List.fold q.djns ~f:(fun djn p ->
+           Formula.orN (List.map djn ~f:pure_approx) :: p ) )
 
 let pure_approx q =
   [%Trace.call fun {pf} -> pf "%a" pp q]
@@ -608,7 +601,7 @@ let fold_dnf ~conj ~disj sjn (xs, conjuncts) disjuncts =
   and split_case pending_splits (xs, conjuncts) disjuncts =
     match Iter.pop pending_splits with
     | Some (split, pending_splits) ->
-        List.fold split ~init:disjuncts ~f:(fun disjuncts sjn ->
+        List.fold split disjuncts ~f:(fun sjn disjuncts ->
             add_disjunct pending_splits sjn (xs, conjuncts) disjuncts )
     | None -> disj (xs, conjuncts) disjuncts
   in
@@ -658,15 +651,15 @@ let rec freshen_nested_xs q =
   (* trim xs to those that appear in the stem and sink the rest *)
   let fv_stem = fv {q with xs= Var.Set.empty; djns= []} in
   let xs_sink, xs = Var.Set.diff_inter q.xs fv_stem in
-  let xs_below, djns =
-    List.fold_map ~init:Var.Set.empty q.djns ~f:(fun xs_below djn ->
-        List.fold_map ~init:xs_below djn ~f:(fun xs_below dj ->
+  let djns, xs_below =
+    List.fold_map q.djns Var.Set.empty ~f:(fun djn xs_below ->
+        List.fold_map djn xs_below ~f:(fun dj xs_below ->
             (* quantify xs not in stem and freshen disjunct *)
             let dj' =
               freshen_nested_xs (exists (Var.Set.inter xs_sink dj.us) dj)
             in
             let xs_below' = Var.Set.union xs_below dj'.xs in
-            (xs_below', dj') ) )
+            (dj', xs_below') ) )
   in
   (* rename xs to miss all xs in subformulas *)
   freshen_xs {q with xs; djns} ~wrt:(Var.Set.union q.us xs_below)
@@ -688,7 +681,7 @@ let rec propagate_context_ ancestor_vs ancestor_ctx q =
   let ancestor_stem = and_ctx_ ancestor_ctx stem in
   let ancestor_ctx = ancestor_stem.ctx in
   exists xs
-    (List.fold djns ~init:ancestor_stem ~f:(fun q' djn ->
+    (List.fold djns ancestor_stem ~f:(fun djn q' ->
          let dj_ctxs, djn =
            List.rev_map_split djn ~f:(fun dj ->
                let dj = propagate_context_ ancestor_vs ancestor_ctx dj in
diff --git a/sledge/src/solver.ml b/sledge/src/solver.ml
index 2cd88460a..3357cbd37 100644
--- a/sledge/src/solver.ml
+++ b/sledge/src/solver.ml
@@ -93,13 +93,11 @@ end = struct
       let us = Option.value us ~default:Var.Set.empty in
       let us =
         Option.fold
-          ~f:(fun us sub -> Var.Set.union (Var.Set.diff sub.Sh.us xs) us)
-          sub ~init:us
+          ~f:(fun sub -> Var.Set.union (Var.Set.diff sub.Sh.us xs))
+          sub us
       in
       let union_us q_opt us' =
-        Option.fold
-          ~f:(fun us' q -> Var.Set.union q.Sh.us us')
-          q_opt ~init:us'
+        Option.fold ~f:(fun q -> Var.Set.union q.Sh.us) q_opt us'
       in
       union_us com (union_us min us)
     in
@@ -655,8 +653,8 @@ let excise_dnf : Sh.t -> Var.Set.t -> Sh.t -> Sh.t option =
   let dnf_subtrahend = Sh.dnf subtrahend in
   Iter.fold_opt
     (Iter.of_list dnf_minuend)
-    ~init:(Sh.false_ (Var.Set.union minuend.us xs))
-    ~f:(fun remainders minuend ->
+    (Sh.false_ (Var.Set.union minuend.us xs))
+    ~f:(fun minuend remainders ->
       [%trace]
         ~call:(fun {pf} -> pf "@[<2>minuend@ %a@]" Sh.pp minuend)
         ~retn:(fun {pf} -> pf "%a" (Option.pp "%a" Sh.pp))
diff --git a/sledge/src/test/equality_test.ml b/sledge/src/test/equality_test.ml
index b1e4faf91..54cdd260d 100644
--- a/sledge/src/test/equality_test.ml
+++ b/sledge/src/test/equality_test.ml
@@ -42,9 +42,7 @@ let%test_module _ =
     let z = Term.var z_
 
     let of_eqs l =
-      List.fold ~init:(wrt, true_)
-        ~f:(fun (us, r) (a, b) -> and_eq us a b r)
-        l
+      List.fold ~f:(fun (a, b) (us, r) -> and_eq us a b r) l (wrt, true_)
       |> snd
 
     let implies_eq r a b = implies r (Term.eq a b)
diff --git a/sledge/src/test/fol_test.ml b/sledge/src/test/fol_test.ml
index 31ef18d97..d3e8ecb98 100644
--- a/sledge/src/test/fol_test.ml
+++ b/sledge/src/test/fol_test.ml
@@ -47,9 +47,9 @@ let%test_module _ =
     let g = Term.mul
 
     let of_eqs l =
-      List.fold ~init:(wrt, empty)
-        ~f:(fun (us, r) (a, b) -> add us (Formula.eq a b) r)
-        l
+      List.fold
+        ~f:(fun (a, b) (us, r) -> add us (Formula.eq a b) r)
+        l (wrt, empty)
       |> snd
 
     let add_eq a b r = add wrt (Formula.eq a b) r |> snd
diff --git a/sledge/src/test/sh_test.ml b/sledge/src/test/sh_test.ml
index 0cd44a65f..11b8ea424 100644
--- a/sledge/src/test/sh_test.ml
+++ b/sledge/src/test/sh_test.ml
@@ -52,7 +52,7 @@ let%test_module _ =
            (Array.map ~f:(fun (siz, seq) -> Term.sized ~siz ~seq) ms))
 
     let of_eqs l =
-      List.fold ~init:emp ~f:(fun q (a, b) -> and_ (Formula.eq a b) q) l
+      List.fold ~f:(fun (a, b) q -> and_ (Formula.eq a b) q) l emp
 
     let%expect_test _ =
       pp
@@ -143,8 +143,8 @@ let%test_module _ =
         ( (  0 = _ ∧ emp)
         ∨ (  ( (  1 = _ = %y_7 ∧ emp) ∨ (  2 = _ ∧ emp) ))
         )
-    
-        ( (  1 = %y_7 ∧ emp) ∨ (  emp) ∨ (  emp) ) |}]
+
+        ( (  emp) ∨ (  1 = %y_7 ∧ emp) ∨ (  emp) ) |}]
 
     let%expect_test _ =
       let q = exists ~$[x_] (of_eqs [(f x, x); (f y, y - !1)]) in