Finding the negative

Version 9.1.
I have the following query that brings back the correct number of lobs still asscociated with data in other tables:

select count(distinct(loid)) from pg_largeobject where (loid in (select content from attachments) or loid in (select content from idw_form_workflow_attachments));

in that the loid's in either table are found. What I need is to find the ones NOT in either table. I have tried using not in both with or/and and various parenthesis. I tried:

select count(distinct(loid)) from pg_largeobject where not ((loid in (select content from attachments) or loid in (select content from idw_form_workflow_attachments)));

This should work since, if the loid is not in the content of either table, it should be F or F = F, negated to T;
which I would think would then count, but I get back 0 instead of the rather large number I should get. This is all a prelude to removing lobs that are no longer referenced.

Any ideas on how to get the inverse of the working query? Thanks