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 balance'_shape2: forall n tl k tr, is_redblack tl Black n -> is_redblack tr Black n -> is_redblack (balance' tl k tr) Red (S n).