def __is_const_str(data: str) -> bool:
    """ Returns true if predicate is a constant string.

    Note: supports constant strings starting with a lowercase letter
    or a number

    """
    # common case: string starts with a lowercase letter
    test_str = data[0].islower()
    # special case: string starts with a number. see '16_bit_one's_complement'
    test_num = data[0].isnumeric() and not data.isnumeric()
    return  (test_str or test_num) and not '@' in data