protocol NListType : SequenceType {
  typealias Element
  init()
  var ar: [Element] { get }
}

extension NListType {
  func generate() -> IndexingGenerator<[Element]> {
    return ar.generate()
  }
  var count: Int { return ar.count }
}

struct Empty<Element> : NListType {
  init() {}
  let ar: [Element] = []
}

struct NList<Element, Tail: NListType where Tail.Element == Element> : NListType {

  init(_ head: Element, _ tail: Tail = Tail()) {
    self.tail = tail
    self.head = head
  }

  init() { fatalError() }

  var ar: [Element] { return [head] + tail.ar }

  let tail: Tail
  let head: Element

}

infix operator |> { associativity right precedence 90 }

func |><E, L : NListType where L.Element == E>(lhs: E, rhs: L) -> NList<E, L> {
  return NList(lhs, rhs)
}