Definition belast {T : Type} (s : seq T) :=
match s with
| [::] => [::]
| h :: s => belast h s
end.

Lemma zip_rcons {S T : Type} (s : seq S) (t : seq T) hs ht : size s = size t ->
zip (rcons s hs) (rcons t ht) = rcons (zip s t) (hs,ht).