[sledge] Add Qset.is_empty

Reviewed By: jvillard

Differential Revision: D20663962

fbshipit-source-id: a0d23d4fc

sledge/lib/import/qset.ml

@@ -71,6 +71,7 @@ struct
     M.fold m' ~init:m ~f:(fun ~key:x ~data:i m -> add m x i)
 
   let map_counts m ~f = M.mapi ~f:(fun ~key ~data -> f key data) m
+  let is_empty = M.is_empty
   let length m = M.length m
   let count m x = match M.find m x with Some q -> q | None -> Q.zero
   let choose = M.choose

sledge/lib/import/qset_intf.ml

@@ -43,6 +43,8 @@ module type S = sig
 
   (* queries *)
 
+  val is_empty : t -> bool
+
   val length : t -> int
   (** Number of elements with non-zero multiplicity. [O(1)]. *)