Definition at_least_one_then_f (o:option elt)(o':option elt') :=
  match o, o' with
   | None, None => None
   | _, _  => f o o'
  end.

Lemma map2_0 : forall m (Hm : sort (@ltk elt) m) m' (Hm' : sort (@ltk elt') m') (x:key), find x (map2 m m') = at_least_one_then_f (find x m) (find x m').