docs: function add(err, req, res) {
    if(err) {
      // TODO: send args length errors
  
      // transaction errors are not queued
      // but the transaction is left open for more
      // command to be queued
      if(err instanceof TransactionError) {
        return res.send(err);
      }
      // last error
      this.error = err;
    }
  
    this.queue.push({err: err, req: req, res: res});
    res.send(null, QUEUED);
  }