docs: function (cb) {
              if (cb) {
                  if (isPromise(cb)) {
                      cb = cb.errback;
                  }
                  if (this.__fired && this.__error) {
                      this.__callNextTick(cb, this.__error);
                  } else {
                      this.__errorCbs.push(cb);
                  }
              }
              return this;
          }