trait _Iter {
  def hasNext: Boolean;

  def map(f: Int => Int) = ();

  def filter(f: Int => Boolean): _Iter = new _BufferedIter {
    def head = 0;
    def hasNext = true;
    // here we should inherit map and filter, as concrete methods
  }
}


trait _BufferedIter extends _Iter {
  def head: Int;
}