I'm not sure that I understand the question correctly. There is a category for HFA already after all - it's classical/kanner's/infantile autism. Based on questionable subjective "evidence" a person with classical autism may get semi-officially categorised as having HFA, MFA or LFA.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett