Module S := FMapList.Make(XOT).
(** The type of dependent maps **)
Definition key := X.t.
Definition OK {A} dom (map : S.t A) := ∀ x, S.In x map <-> Dom.In x dom.
Definition t := fun A dom => { map : S.t A | OK dom map}.
Instance OK_compat A : Proper (Dom.eq ==> @S.Equal A ==> iff) OK.
(** ***  Operations on maps **)
Definition empty : forall A, t A Dom.empty := fun A => exist (OK Dom.empty) (@S.empty A) (empty_OK A).
Definition is_empty (A : Type) dom (m : t A dom) := Dom.equal dom Dom.empty.
Definition mem (A : Type) dom (x : key) (m : t A dom) := Dom.mem x dom.
Definition find (A : Type) dom (x : key) (m : t A dom) := S.find x (proj1_sig m).
Definition domain (A : Type) dom (m : t A dom) := dom.

(** [add] **)
Lemma add_OK : forall A dom x v (m : t A dom), OK (Dom.add x dom) (S.add x v (proj1_sig m)).