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_leaf_path : forall v t p (Hfind : find_leaf v t = Some p) (Hwf : wf_btree t), leaf_path t (removelast p).