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 infrared_append:
forall tl tr n, is_redblack tl Black n -> is_redblack tr Black n ->
infrared (append (tl,tr)) n.