Fixpoint from_vec (A: Set) n: Vector.t A (S n) -> L A :=
  match n return Vector.t A (S n) -> L A with
  | 0 => fun v => one (vec.head v)
  | _ => fun v => cons (vec.head v) (from_vec (vec.tail v))
  end.

Lemma from_vec_to_plain (A: Set) n (v: Vector.t A (S n)): to_plain (from_vec v) = vec.to_list v.