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."""
        self.validate()
        return u'({predicate} ? {if_true} : {if_false})'.format(
            predicate=self.predicate.to_gremlin(),
            if_true=self.if_true.to_gremlin(),
            if_false=self.if_false.to_gremlin())