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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 invalid_foot_to_spondee(self, feet: list, foot: str, idx: int) -> str: """ In hexameters, a single foot that is a unstressed_stressed syllable pattern is often just a double spondee, so here we coerce it to stressed. :param feet: list of string representations of meterical feet :param foot: the bad foot to correct :param idx: the index of the foot to correct :return: corrected scansion -UU----U----UU """

        new_foot = foot.replace(self.constants.UNSTRESSED, self.constants.STRESSED)
        feet[idx] = new_foot
        return "".join(feet)