Definition Equal m m' := forall y, find y m = find y m'.Definition Equiv (eq_elt:elt->elt->Prop) m m' :=
   (forall k, In k m <-> In k m') /\
   (forall k e e', MapsTo k e m -> MapsTo k e' m' -> eq_elt e e').Definition Equivb cmp := Equiv (Cmp cmp).

Lemma map2_2 : forall (elt elt' elt'':Type)(m: t elt)(m': t elt') (x:key)(f:option elt->option elt'->option elt''), In x (map2 f m m') -> In x m \/ In x m'.