Ltac map2_aux :=
 match goal with
  | H : In ?x _ \/ In ?x ?m,
    H' : find ?x ?m = find ?x ?m', B:bst ?m, B':bst ?m' |- _ =>
    destruct H; [ intuition_in; order |
                  rewrite <-(find_in_equiv B B' H'); auto ]
 end.

Lemma map2_opt_1 : forall m m' y, bst m -> bst m' -> In y m \/ In y m' -> find y (map2_opt m m') = f0 y (find y m) (find y m').