Definition option_cons (A:Type)(k:key)(o:option A)(l:list (key*A)) :=
  match o with
   | Some e => (k,e)::l
   | None => l
  end.Definition map2 m m' :=
  let m0 : t oee' := combine m m' in
  let m1 : t (option elt'') := map (fun p => f (fst p) (snd p)) m0 in
  fold_right_pair (option_cons (A:=elt'')) nil m1.

Lemma map2_NoDup : forall m (Hm:NoDupA (@eqk elt) m) m' (Hm':NoDupA (@eqk elt') m'), NoDupA (@eqk elt'') (map2 m m').