Fixpoint zip (s : seq S) (t : seq T) {struct t} :=
  match s, t with
  | x :: s', y :: t' => (x, y) :: zip s' t'
  | _, _ => [::]
  end.Definition unzip1 := map (@fst S T).Definition unzip2 := map (@snd S T).

Lemma size2_zip s t : size t <= size s -> size (zip s t) = size t.