Definition is_red_or_empty t :=
match t with T Black _ _ _ => False | _ => True end.
Definition is_black t :=
match t with T Black _ _ _ => True | _ => False end.

Lemma append_is_redblack:
forall tl tr n, is_redblack tl Red n -> is_redblack tr Red n ->
is_redblack (append (tl,tr)) Black n.