code: def _find_consonant_cluster(self, letters: List[str]) -> List[int]:
          
          for idx, letter_group in enumerate(letters):
              if self._contains_consonants(letter_group) and not self._contains_vowels(letter_group):
                  return [idx]
          return []
docs: Find clusters of consonants that do not contain a vowel.
          :param letters:
          :return: