sclass AllOnAll<A, B> {
  new L<A> aList;
  new L<B> bList;
  LPair<L<A>, L<B>> todo = new LinkedList;
  LPair<A, B> todo2 = new LinkedList;

  void newA(A a) {
    add(aList, a);
    addPair(todo, ll(a), cloneList(bList));
  }
  
  void newB(B b) {
    add(bList, b);
    addPair(todo, cloneList(aList), ll(b));
  }
  
  Pair<A, B> next() {
    while (empty(todo2)) {
      if (empty(todo)) null;
      addAll(todo2, outerProduct_pairs(popFirst(todo)));
    }
    ret popFirst(todo2);
  }
}