Definition remove_In (X: Set) n (v: Vector.t X (S n)) x i (p: List.In x (remove v i)): List.In x v
  := incl_In x p (SkipList_incl (SkipList_remove i v)).

(* Specifications: *)
Lemma trans_plus_n_Sm n m: n + S m = S (n + m).