associatedtype SubSequence : Collection = Slice<Self> where Self.Element == Self.SubSequence.Element, Self.SubSequence == Self.SubSequence.SubSequenceassociatedtype SubSequence : Collection = Slice<Self> where Self.Element == Self.SubSequence.Element, Self.SubSequence == Self.SubSequence.SubSequenceAny まではいかなくて、↑にマッチする最も抽象的な型？