unique constraint on 2 columns

[Jonathan Vanasco <postgres(at)2xlp(dot)com>]

I need a certain unique constraint in pg that i can't figure out.

Given:

	create table test_a (
		id serial ,
		name_1 varchar(32) ,
		name_2 varchar(32)
	);

I need name_1 and name_2 to both be unique so that:
	name_1 never appears in name_1 or name_2
	name_2 never appears in name_2 or name_1


a standard 2 column unique index / constraint will not accomplish this.