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 nth_error_succeeds : forall {A} n (l : list A) (Hlt : (n < length l)%nat), exists x, nth_error l n = Some x.