sclass AllOnAllWithOperation<A, B> extends AllOnAll<A, B> implements Steppable {
  swappable void operation(A a, B b) {}
  
  *() {}
  *(IVF2<A, B> *operation) {}

  public bool step() {
    Pair<A, B> p = next();
    if (p == null) false;
    operation(p.a, p.b);
    true;
  }
}