assignment ::= relation_name assign expression |
            relation_name param_start attribute_list param_stop
            assign expression

def assignment(self):
        """
        assignment ::= relation_name assign expression |
            relation_name param_start attribute_list param_stop
            assign expression
        """
        lhs = Group(self.relation_name +
                    Optional(self.parenthesize(self.attribute_list)))
        return Group(lhs + Keyword(self.syntax.assign_op) + self.expression)