def p_attr_field(self, p):
        """attr_field : ID EQ primitive NL
                      | ID EQ tag_ref NL"""
        if p[3] is NullToken:
            p[0] = AstAttrField(
                self.path, p.lineno(1), p.lexpos(1), p[1], None)
        else:
            p[0] = AstAttrField(
                self.path, p.lineno(1), p.lexpos(1), p[1], p[3])