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

        >>> print(HexameterScanner().invalid_foot_to_spondee(
        ... ['-UU', '--', '-U', 'U-', '--', '-UU'],'-U', 2))  # doctest: +NORMALIZE_WHITESPACE
        -UU----U----UU
        """
        new_foot = foot.replace(self.constants.UNSTRESSED, self.constants.STRESSED)
        feet[idx] = new_foot
        return "".join(feet)