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.