Variable f : option elt -> option elt' -> option elt''.
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').