docs: def is_deaf(self):
          """
          Глухая ли согласная.
          """
          if not self.is_consonant():
              return False
          if self.letter in self.forever_deaf:
              return True
          if self.letter in self.forever_sonorus:
              return False
          if self.__forsed_sonorus:
              return False
          if self.__forsed_sonorus is False:
              return True
          for _, df in self.sonorus_deaf_pairs:
              if self.letter == df:
                  return True
          return False