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

Lemma combine_l_1 :
forall m (Hm:NoDupA (@eqk elt) m) m' (Hm':NoDupA (@eqk elt') m')(x:key),
find x (combine_l m m') = at_least_left (find x m) (find x m').