trigger to enforce FK with nulls?

[PostgreSQL 8.1.3 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.0.1]

I'm starting to use lots of foreign key constraints to keep my
data clean. In one case, however, I need to allow null values
for the key. E.g.:

create table opset_steps(opset text, step text, step_num int);
create table steps(run text, step text, opset text, user text, step_num int);

The constraint on steps should be:
steps.(opset,step) must be found in opset_steps.(opset,step) UNLESS steps.opset is null.

I started to put dummy rows in opset_steps with opset='NO-OPSET' for each
step with no real opset, but this looks really kludgy.

Since foreign keys per se can't do this, I presume the way
is to use triggers, though I have not used triggers before. Any
suggestions, caveats?

Opset_steps has only 4400 rows, and inserts to steps are not real
frequent, so I don't *think* performace should be a problem. No
updates are ever made to steps.opset and steps.step, or to
opset_steps.(opset,step)
[though updates are often made to *other* fields of steps].

-- George Young

--
"Are the gods not just?" "Oh no, child.
What would become of us if they were?" (CSL)