How to use a cross column exclude constraint

Hi All,

Schema:
CREATE TABLE record ( id uuid primary key default ..., ...);CREATE TABLE record_pointer ( id uuid primary key default ..., record_a_id uuid not null references record (id), record_b_id uuid not null references record (id), ...);

I am trying to create an exclude constraint to make both record_a_id and record_b_id unique table wide, so that each row from "record" can ever be referenced once in "record_pointers". 

Eg. if I add a record_pointer row that has "this-is-random-uuid" as record_a_id, the value "this-is-random-uuid" can never be in record_a_id or record_b_id in any other row.

Thank you in advance!