Ltac nonify e :=
replace e with (@None elt) by
(symmetry; rewrite not_find_iff; auto; intro; order).

(* Specifications: *)
Lemma map_option_find : forall (m:t elt)(x:key),
bst m ->
find x (map_option f m) =
match (find x m) with Some d => f x d | None => None end.