diff --git a/sledge/src/trm.ml b/sledge/src/trm.ml
index 381ccf61b..93ad112e4 100644
--- a/sledge/src/trm.ml
+++ b/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)
diff --git a/sledge/src/trm.mli b/sledge/src/trm.mli
index f9b453b6b..67120bfe0 100644
--- a/sledge/src/trm.mli
+++ b/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 *)