Mass Allocation Formula - can I omit specific code combination from cost pool?

Hi All

I have a requirement to omit a specific code combination from  the cost-pool (line A). We're on release 11.5.10

Example of our existing cost pool line A; each segment is a parent that includes ALL companies, ALL departments, ALL facilities, ALL natural accounts:

     COALL-DEPALL-FACALL-ACTALL

We need to exclude from the above:

     CO1-DEP1-FAC1-ACT1

I cannot determine how to do this. If we create a parent that excludes,for example, CO1 from parent COALL, then we incorrectly exclude all code combinations from the cost pool that have CO1.

Any ideas how to accomplish this using a mass allocation formula? I've read GL user guide, white papers, postings on this board, etc.