// 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 a;
  volatile long b;

  void setA(long a) { this.a = a; itemReceived(); }
  void setB(long b) { this.b = b; itemReceived(); }
  
  LongConsumer setA aka setterForA() { ret a -> setA(a); }
  LongConsumer setB aka setterForB() { ret b -> setB(b); }
  
  void itemReceived {
    if (incCounter() == 2)
      complete(a, b);
  }
  
  // can override or swap
  swappable void complete(long a, long b) {}
}

Began life as a copy of #1035570