why commutator doesn't work?

Hi

I try to create operator + for varchar and integer with Oracle behave.

create or replace function sum(varchar, int)
returns int as $$ select $1::int + $2 $$ language sql;

create operator + (function = sum, leftarg = varchar, rightarg = int, commutator = +);

create table foo2(a varchar);

insert into foo2 values('10');

select a + 1 from foo2; -- it is ok

but

select 1 + a from foo2; -- fails

ERROR:  operator is only a shell: integer + character varying
LINE 1: select 1 + a  from foo2;

Why? This should be solved by COMMUTATOR = +

Regards

Pavel