Module L := FMapList.Raw E.
(** Not exactly pretty nor perfect, but should suffice as a first naive implem. Anyway, map2 isn't in Ocaml... *)
Definition anti_elements (A:Type)(l:list (key*A)) := L.fold (@add _) l (empty _).
Definition map2 (A B C:Type)(f:option A->option B -> option C)(m:t A)(m':t B) : t C :=
anti_elements (L.map2 f (elements m) (elements m')).

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