Return a unicode object with the Gremlin representation of this expression.

def to_gremlin(self):
        """Return a unicode object with the Gremlin representation of this expression."""
        translation_table = {
            u'size': u'count()',
        }
        gremlin_operator = translation_table.get(self.operator)
        if not gremlin_operator:
            raise AssertionError(u'Unrecognized operator used: '
                                 u'{} {}'.format(self.operator, self))

        template = u'{inner}.{operator}'
        args = {
            'inner': self.inner_expression.to_gremlin(),
            'operator': gremlin_operator,
        }
        return template.format(**args)