code: def _mutate(self):
          '''
          Mutate enclosed fields
          '''
          for i in range(self._field_idx, len(self._fields)):
              self._field_idx = i
              if self._current_field().mutate():
                  return True
              self._current_field().reset()
          return Falsf _mutate(self):
          '''
          Mutate enclosed fields
          '''
          for i in range(self._field_idx, len(self._fields)):
              self._field_idx = i
              if self._current_field().mutate():
                  return True
              self._current_field().reset()
          return False
docs: Mutate enclosed fields