extension Sequence where SubSequence : Sequence, SubSequence.Iterator.Element == Iterator.Element, SubSequence.SubSequence == SubSequence { public func prefix(_ maxLength: Int) -> AnySequence<Iterator.Element> { } }

これもしかしてSubSequenceがAnySequenceであることを示唆してる気がしてきた(AnySequence.SubSequence==AnySequence) (edited)