sclass AllOnAllWithUpdates<A, B> {
  long aCounter, bCounter;
  new TreeMap<Long, A> aList;
  new TreeMap<Long, B> bList;
  LPair<LongRange> todo = new LinkedList;
  LPair<Long> todo2 = new LinkedList;

  synchronized void newA(A a) {
    long c = aCounter++;
    aList.put(c, a);
    addPair(todo, longRange_one(c), longRange(0, bCounter));
  }
  
  synchronized void newB(B b) {
    long c = bCounter++;
    bList.put(c, b);
    addPair(todo, longRange(0, aCounter), longRange_one(c));
  }
  
  synchronized void updateA(A a) {
    removeValue(aList, a);
    newA(a);
  }
  
  synchronized void updateB(B b) {
    removeValue(bList, b);
    newB(b);
  }
  
  synchronized Pair<A, B> next() {
    while licensed {
      while (empty(todo2)) {
        if (empty(todo)) null;
        Pair<LongRange> p = popFirst(todo);
        addAll(todo2, outerProduct_pairs(keys(subMap_longRange(aList, p.a)), keys(subMap_longRange(bList, p.b))));
      }
      Pair<Long> p = popFirst(todo2);
      A a = aList.get(p.a);
      B b = bList.get(p.b);
      if (a != null && b != null) ret pair(a, b);
    }
    null;
  }
}

Began life as a copy of #1023892