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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def correct_invalid_start(self, scansion: str) -> str: """ The third syllable of a hendecasyllabic line is long, so we will convert it. :param scansion: scansion string :return: scansion string with corrected start - U - U U - U - U - U """

        mark_list = string_utils.mark_list(scansion)
        vals = list(scansion.replace(" ", ""))
        corrected = vals[:2] + [self.constants.STRESSED] + vals[3:]
        new_line = list(" " * len(scansion))
        for idx, car in enumerate(corrected):
            new_line[mark_list[idx]] = car
        return "".join(new_line)