Section Map.
Variables (n0 : nat) (T1 : Type) (x1 : T1).
Variables (T2 : Type) (x2 : T2) (f : T1 -> T2).
Fixpoint map s := if s is x :: s' then f x :: map s' else [::].

Lemma map_cons x s : map (x :: s) = f x :: map s.