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

Lemma pairmap_rcons {T T0 : Type} (f : T -> T -> T0) (s : seq T) h0 h x0 :
pairmap f x0 (rcons (rcons s h0) h) = rcons (pairmap f x0 (rcons s h0)) (f h0 h).