// assumes that each set function is called exactly once
// (but it doesn't matter which thread you call each of them from.)
sclass LongPairCollector > AtomicCounter {
  volatile long valueOfA;
  volatile long valueOfB;
  
  final LongConsumer a = l1 setA;
  final LongConsumer b = l1 setB;

  void setA(long a) { valueOfA = a; itemReceived(); }
  void setB(long b) { valueOfB = b; itemReceived(); }
  
  LongConsumer setA aka setterForA() { ret a; }
  LongConsumer setB aka setterForB() { ret b; }
  
  void itemReceived {
    if (incCounter() == 2)
      complete(valueOfA, valueOfB);
  }
  
  // override me
  void complete(long a, long b) {}
}