code: @SuppressWarnings({ "unchecked", "rawtypes" })
      public void add(Future future) {
          checkAddAllowed();
          checkInEventLoop();
          ++expectedCount;
          future.addListener(listener);
      }
docs: Adds a new future to be combined. New futures may be added until an aggregate promise is added via the
  {@link PromiseCombiner#finish(Promise)} method.
  
  @param future the future to add to this promise combiner