#[local]
Hint Resolve concat_bst : core.

Lemma concat_find : forall m1 m2 y, bst m1 -> bst m2 ->
(forall y1 y2, In y1 m1 -> In y2 m2 -> X.lt y1 y2) ->
find y (concat m1 m2) =
match find y m2 with Some d => Some d | None => find y m1 end.