Simply put, can you use a Default as a complementary skill? For instance, if you've got COG 40 and you need to do some Viral Marketing (with your Professional Skill: Viral Marketing, which might or might not also be a default), can you then use your COG of 40 for the values of your Academics: Psychology, Academics: Memetics, and Professional Skill: Social Engineering?
If so, it'd definately go a long way to backing up the statements about people with high (31+) Aptitudes being hypercompetent posthumans.
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+1 r-Rep , +1 @-rep