code: public void onChildThreadLaunchBegin()
    {
      _threadCount.incrementAndGet();
      
      int startCount = _startingCount.decrementAndGet();
  
      if (startCount < 0) {
        _startingCount.set(0);
        
        new IllegalStateException().printStackTrace();
      }
  
      _createCountTotal.incrementAndGet();
      
      wakeIfLowIdle();
    }
docs: Callback from the launched thread's run().
  
  Must _not_ be called by any other method, including other spawning.