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

  void addA(A a) { newA(a); }
  void addAIfNotNull(A a) { if (a != null) newA(a); }
  
  synchronized void newA(A a) {
    add(aList, a);
    addPair(todo, intRange_last(aList), intRange(0, l(bList)));
  }
  
  synchronized void newAs(Iterable<A> l) {
    fOr (A a : l) newA(a);
  }
  
  void addB(B b) { newB(b); }
  void addBIfNotNull(B b) { if (b != null) newB(b); }
  
  synchronized void newB(B b) {
    add(bList, b);
    addPair(todo, intRange(0, l(aList)), intRange_last(bList));
  }
  
  synchronized void newBs(Iterable<B> l) {
    fOr (B b : l) newB(b);
  }
  
  public synchronized Pair<A, B> next() {
    while (empty(todo2)) {
      if (empty(todo)) null;
      Pair<IntRange> p = popFirst(todo);
      addAll(todo2, outerProduct_pairs(subList(aList, p.a), subList(bList, p.b)));
    }
    ret popFirst(todo2);
  }
  
  synchronized L cloneBList() {
    ret clonedList(bList);
  }
  
  L<A> getAs() { ret aList; }
  L<B> getBs() { ret bList; }
}