AList as Set

Traits:

• AList with cases ANil, ACons and AMany (with AMany =ANil ≀ ACons)
• AOption with cases ANone, ASome and AMaybe (with AMaybe = ANone ≀ ASome)
• ABool with cases ATrue, AFalse and AUnknown (with AUnknown = ATrue ≀ AFalse)

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Method signatures:

• aHead (l: Set[AList]): Set[Set[AOption[AInt]]]
• aTail (l: Set[AList]): Set[Set[AOption[AList]]]
• isNil (l: Set[AList]): Set[Set[ABool]]