protocol Collection {
  associatedType Segments : Collection where Segments.Element : Collection, Segments.Element.Element == Element = CollectionOfOne<Self>
  var segments: Segments? { get }
  // ...existing definition...
}
extension Collection where Segments == CollectionOfOne<Self> {
  var segments: Segments? { return nil }
}
// ...refined constraints for BidirectionalCollection, 
// RandomAccessCollection, and MutableCollection...