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_1 : forall (A B C:Type)(m: t A)(m': t B)(x:key) (f:option A->option B ->option C), In x m \/ In x m' -> find x (map2 f m m') = f (find x m) (find x m').