def p_topic(self, p):
        """topic : LBRACKET morpheme RBRACKET
                | LBRACKET morpheme RBRACKET literal_list
                | LBRACKET morpheme TIMES morpheme RBRACKET
                | LBRACKET morpheme TIMES morpheme RBRACKET literal_list"""

        if len(p) == 4:
            p[0] = Topic(root=tuple(p[2]), flexing=())
        elif len(p) == 5:
            p[0] = Topic(root=tuple(p[2]), flexing=(), literals=p[4])
        elif len(p) == 6:
            p[0] = Topic(root=tuple(p[2]), flexing=tuple(p[4]))
        else:
            p[0] = Topic(root=tuple(p[2]), flexing=tuple(p[4]), literals=p[6])
topic : LBRACKET morpheme RBRACKET
                | LBRACKET morpheme RBRACKET literal_list
                | LBRACKET morpheme TIMES morpheme RBRACKET
                | LBRACKET morpheme TIMES morpheme RBRACKET literal_list