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 disjoint_cons_r :
forall A B x a (E : list (X.t*A)) (F : list (X.t*B)),
disjoint F ((x, a) :: E) <-> ~ In x (dom F) /\ disjoint E F.