sclass AllOnAllOperation<A, B> implements Steppable {
  AllOnAll<A, B> allOnAll;
  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;
  }
}