@classmethod
def get_label(cls):
    """
        return the next label
        """
    code = cls.last_label
    cls.last_label += 1
    if code < 26:
        label = chr(97 + code)
    elif code < 26 * 26:
        label = chr(96 + code // 26) + chr(97 + code % 26)
    else:
        a = code % 26
        b = code // 26
        label = chr(96 + b // 26) + chr(97 + b % 26) + chr(97 + a)
    return label, code
