Notation oee' := (option elt * option elt')%type.Definition combine_l (m:t elt)(m':t elt') : t oee' :=
  mapi (fun k e => (Some e, find k m')) m.Definition combine_r (m:t elt)(m':t elt') : t oee' :=
  mapi (fun k e' => (find k m, Some e')) m'.Definition fold_right_pair (A B C:Type)(f:A->B->C->C) :=
  List.fold_right (fun p => f (fst p) (snd p)).Definition combine (m:t elt)(m':t elt') : t oee' :=
  let l := combine_l m m' in
  let r := combine_r m m' in
  fold_right_pair (add (elt:=oee')) r l.

Lemma combine_NoDup : forall m (Hm:NoDupA (@eqk elt) m) m' (Hm':NoDupA (@eqk elt') m'), NoDupA (@eqk oee') (combine m m').