[sledge] Add Trm.fv

Reviewed By: da319

Differential Revision: D24532353

fbshipit-source-id: 506105bd1

sledge/src/trm.ml

@@ -492,3 +492,7 @@ let iter_subtrms e ~f =
   | Concat xs | Record xs | Apply (_, xs) -> Array.iter ~f xs
 
 let subtrms e = Iter.from_labelled_iter (iter_subtrms e)
+
+(** Query *)
+
+let fv e = Var.Set.of_iter (vars e)

sledge/src/trm.mli

@@ -95,6 +95,7 @@ val map : t -> f:(t -> t) -> t
 
 val seq_size_exn : t -> t
 val seq_size : t -> t option
+val fv : t -> Var.Set.t
 
 (** Traverse *)