#[local]
Hint Resolve map2_opt_bst : core.
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.
Ltac nonify t :=
match t with (find ?y (map2_opt ?m ?m')) =>
replace t with (@None elt'');
[ | symmetry; rewrite not_find_iff; auto; intro;
destruct (@map2_opt_2 m m' y); auto; order ]
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').