Fixpoint wf_btree_over t :=
match t with BNode keys children =>
(length keys = 2 * d + 1)%nat /\
(children = [] \/ length children = S (length keys) /\ Forall wf_btree_aux children)
end.

Lemma find_index_le : forall tgt keys i
(Hfind : find_index tgt keys = NotFound i),
(i <= length keys)%nat.