Return a list of "user friendly" labels.
        
        :return     <list> [ <str>, .. ]

def labels(self):
        """
        Return a list of "user friendly" labels.
        
        :return     <list> [ <str>, .. ]
        """
        return [self._labels.get(value) or text.pretty(key)
                for key, value in sorted(self.items(), key=lambda x: x[1])]