code: private boolean isSenderRecvPair(Event senderCnd, Event recvCnd) {
      if (senderCnd.isSend() && recvCnd.isRecv() && senderCnd.match(recvCnd)) {
        ArrayList<Event> recvs = _sends.get(senderCnd);
        if (recvs.isEmpty() || senderCnd.packH2O()==null ) {
          for (Event e : recvs)
            if (e._nodeId == recvCnd._nodeId)
              return false;
          return true;
        }
      }
      return false;
    }
docs: Check whether two events can be put together in sender/recv relationship.
  
  Events must match, also each sender can have only one receiver per node.
  
  @param senderCnd
  @param recvCnd
  @return