• Returns elements of the primary set that are not found in the secondary set
• Both sets must be real sets, not organisms (in contrast to COMMON-ORTHOLOGS-OF)

Example:
  (DEFINE heterocystous-cb AS (Npun A7120 Avar))
  ::
  (SET-DIFFERENCE (COMMON-ORTHOLOGS-OF heterocystous-cb PRIMARY A7120)
                  (COMMON-ORTHOLOGS-OF (A7120 Tery)))

          [Result is a list of proteins within Anabaena PCC7120 with orthologs in all heterocystous
            cyanobacteria but no orthologs in Trichodesmium eryhraeum.