def is_derivative_symbol(expr, t_symbol=None):
    """
    Returns whether expr is a derivative symbol (w.r.t. t)

    :param expr:
    :param t_symbol:
    :return: True or False
    """
    if t_symbol is not None:
        raise NotImplementedError
    if hasattr(expr, 'difforder') and expr.difforder > 0:
        return True
    else:
        return False
