Foreign Keys and Inheritance

If I have table A, which contains a primary key,
and table B which inherits from A.

How can I Create a Foreign Key on table C that references A, but will
also pass if a record in inserted in to B.

for instance:

create table A
(
        prim_key char(20) not null primary key
);

create table B
(
) INHERITS A;


create table C
(
        data char(2) not null primary key,
        constraint fk_C FOREIGN KEY ( data ) REFERENCES A ( prim_key ) ON DELETE
CASCADE ON UPDATE CASCADE
)



With this setup, my record must be inserted in to A or I'll get a
referential integrity problem when I insert in to C.

I want to be able to insert in to A, B , or another table inherited from A,
and have table C recognize that as it's foreign key.

Thanks