Convert to the next higher level summary level

def promote(self, level=None):
        """Convert to the next higher level summary level"""

        if level is None:

            if len(self.fields) < 2:
                if self.level in ('region', 'division', 'state', 'ua'):
                    cls = self.get_class('us')
                else:
                    return None
            else:
                cls = self.get_class(self.fields[-2])
        else:
            cls = self.get_class(level)

        d = dict(self.__dict__.items())
        d['sl'] = self.sl

        return cls(**d)