Module Make
(X : UsualDecidableType)
(KeySet : FSetInterface.WSfun X).
(* SCW: Make an instance of EqDef_eq for X so that we can use the "==" in [get] below. *)
Instance EqDec_of_X : @EqDec X.t eq eq_equivalence.

Lemma in_app_iff :
List.In x (l1 ++ l2) <-> List.In x l1 \/ List.In x l2.