abstract sclass Convergent<A> extends ItIt<A> {
  A value;
  bool stepped, done;
  
  public bool hasNext() {
    if (done) false;
    
    if (!stepped) {
      stepped = true;
      step();
    }
    
    ret !done;
  }
  
  public A next() {
    assertTrue(hasNext());
    stepped = false;
    ret value;
  }
  
  abstract void step();
}