@staticmethod
def _convert_number_to_subscript(num):
    """
        Converts number into subscript

        input = ["a", "a1", "a2", "a3", "be2", "be3", "bad2", "bad3"]
        output = ["a", "a₁", "a₂", "a₃", "be₂", "be₃", "bad₂", "bad₃"]

        :param num: number called after sign
        :return: number in subscript
        """
    subscript = ''
    for character in str(num):
        subscript += chr(8320 + int(character))
    return subscript
