code: RPC.RPCCall record_task( RPC.RPCCall rpc ) {
      // Task removal (and roll-up) suffers from classic race-condition, which we
      // fix by a classic Dekker's algo; a task# is always in either the _work
      // HashMap, or rolled-up in the _removed_task_ids counter, or both (for
      // short intervals during the handoff).  We can never has a cycle where
      // it's in neither or else a late UDP may attempt to "resurrect" the
      // already completed task.  Hence we must always check the "removed ids"
      // AFTER we insert in the HashMap (we can check before also, but that's a
      // simple optimization and not sufficient for correctness).
      final RPC.RPCCall x = _work.putIfAbsent(rpc._tsknum,rpc);
      if( x != null ) return x;   // Return pre-existing work
      // If this RPC task# is very old, we just return a Golden Completed task.
      // The task is not just completed, but also we have already received
      // verification that the client got the answer.  So this is just a really
      // old attempt to restart a long-completed task.
      if( rpc._tsknum > _removed_task_ids.get() ) return null; // Task is new
      _work.remove(rpc._tsknum); // Bogus insert, need to remove it
      return _removed_task;      // And return a generic Golden Completed object
    }
docs: also ACKACK'd.