End map2.
Definition map2 (elt elt' elt'':Type)(f:option elt->option elt'->option elt'') :=
_map2 (fun o1 o2 => match o1,o2 with None,None => None | _, _ => f o1 o2 end).

(* Specifications: *)
Lemma map2_1 : forall (elt elt' elt'':Type)(m: t elt)(m': t elt')
(x:key)(f:option elt->option elt'->option elt''),
In x m \/ In x m' ->
find x (map2 f m m') = f (find x m) (find x m').
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'.