'If one of the transformations is not defined it is expected to
be the mirror image of the other.'

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()])
