private static void verifyProperty4(Node<?> n) {
    if (!isBlack(n)) {
      assert isBlack(n.getLeft());
      assert isBlack(n.getRight());
      assert isBlack(n.getParent());
    }
    if (n == null) return;
    verifyProperty4(n.getLeft());
    verifyProperty4(n.getRight());
  }
private static void verifyProperty4(Node<?> n)