code: def adjust(self):
          
  
          if self._fro is None and self._to is not None:
              self._fro = dict(
                  [(value.lower(), key) for key, value in self._to.items()])
          if self._to is None and self._fro is not None:
              self._to = dict(
                  [(value.lower(), key) for key, value in self._fro.items()])
docs: If one of the transformations is not defined it is expected to
          be the mirror image of the other.