Section Scan.
Variables (T1 : Type) (x1 : T1) (T2 : Type) (x2 : T2).
Variables (f : T1 -> T1 -> T2) (g : T1 -> T2 -> T1).
Fixpoint pairmap x s := if s is y :: s' then f x y :: pairmap y s' else [::].

Lemma zip_rcons s1 s2 z1 z2 :
size s1 = size s2 ->
zip (rcons s1 z1) (rcons s2 z2) = rcons (zip s1 s2) (z1, z2).