// persistable (if you set "operation", make sure it's with a

// persistable object)

sclass AllOnAllOperation<A, B> implements Steppable {

  Producer<Pair<A, B>> allOnAll; // e.g. an instance of AllOnAll<A, B>

  *() {}

  *(Producer<Pair<A, B>> *allOnAll, IVF2<A, B> *operation) {}

  persistent swappable void operation(A a, B b) {}

  public bool step() {

    Pair<A, B> p = allOnAll.next();

    if (p == null) false;

    operation(p.a, p.b);

    true;

  }

}