Fixpoint Tmap (T: Tree A) : Tree B :=
match T with
Tempty => Tempty
| Branch0 t1 t2 => Branch0 (Tmap t1) (Tmap t2)
| Branch1 a t1 t2 => Branch1 (f a) (Tmap t1) (Tmap t2)
end.

Lemma Tmap_Tadd: forall i a T, Tmap (Tadd i a T) = Tadd i (f a) (Tmap T).