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)).
Definition add {A : Type} {dom : Dom.t} (x : key) (v : A) (m : @t A dom) : @t A (Dom.add x dom) :=
exist (OK (Dom.add x dom)) (S.add x v (proj1_sig m)) (add_OK x v m).