ListZipper.swift

  /// Map a function to the tracks
  func map<R>(f: (T) -> R) -> ListZipper<R> {
    var zipper = ListZipper<R>()
    zipper.leftList = leftList.map(f)
    zipper.cursor = cursor.map(f)
    zipper.rightList = rightList.map(f)
    return zipper
  }