def p_boolean_expr(self, p):
        '''boolean_expr : expr AND expr
                        | expr AMPERSAND expr
                        | expr OR expr
                        | expr IMPLY expr
                        | expr EQUIV expr
                        | NOT expr %prec UMINUS
                        | bool_type'''
        if len(p) == 4:
            p[0] = (p[2], (p[1], p[3]))
        elif len(p) == 3:
            p[0] = (p[1], (p[2],))
        elif len(p) == 2:
            p[0] = ('boolean', p[1])