Corollary insert_at_wf' : forall v p t t' (Hinsert : insert_at v t p = Some t')
      (Hwf : wf_btree t) (Hp : leaf_path t (removelast p)),
      wf_btree t' \/ wf_btree_over t'.

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