// 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;
  }
}

Began life as a copy of #1023892