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

Lemma combine_1 :
forall m (Hm : sort (@ltk elt) m) m' (Hm' : sort (@ltk elt') m') (x:key),
find x (combine m m') = at_least_one (find x m) (find x m').