code: def invalid_foot_to_spondee(self, feet: list, foot: str, idx: int) -> str:
          
          new_foot = foot.replace(self.constants.UNSTRESSED, self.constants.STRESSED)
          feet[idx] = new_foot
          return "".join(feet)
docs: 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