searching bison guru - grouping sets implementation

Hello

I trying to implement GROUPING SETS feature. But there is basic
difference between PostgreSQL and ANSI. Pg allows expressions, ANSI
only column reference. I have syntax:

group_clause:
                        GROUP_P BY grouping_element_list
                        | /*EMPTY*/
                ;

grouping_element_list:
                        grouping_element
                                {
                                        $$ = list_make1($1);
                                }
                        | grouping_element_list ',' grouping_element
                                {
                                        $$ = lappend($1, $3);
                                }
                ;

grouping_element:
                        ordinary_grouping_set
                                {
                                }
                        | ROLLUP '(' ordinary_grouping_set_list ')'
                                {
                                }
                        | CUBE '(' ordinary_grouping_set_list ')'
                                {
                                }
                        | GROUPING SETS '(' grouping_element_list ')'
                                {
                                }
                        | '(' ')'
                                {
                                }
                ;


ordinary_grouping_set:
                    grouping_column_ref
                                {
                                }
                    | '(' grouping_ref_list ')'
                                {
                                }
                ;

grouping_ref_list:
                    grouping_column_ref
                                {
                                }
                    | grouping_ref_list ',' grouping_column_ref
                                {
                                }
                ;

ordinary_grouping_set_list:
                    ordinary_grouping_set
                                {
                                }
                    |  ordinary_grouping_set_list ',' ordinary_grouping_set
                                {
                                }
                ;

grouping_column_ref:
                        columnref
                                {}
                        | Iconst
                                {}
                ;
     ;

this works well, but it is ANSI compliant not pg compliant

after change:
grouping_column_ref:
                        a_expr
                                {}
                ;

I getting
[pavel@localhost parser]$ bison  gram.y
gram.y: conflicts: 1 shift/reduce, 1 reduce/reduce

so I cannot find any way to remove shift/reduce.

any ideas?