code: public static <T> CombinedDownloadTask<T> single(DownloadTask<T> task) {
  		Objects.requireNonNull(task);
  		return new SingleCombinedTask<T>(task);
  	}
docs: Creates a CombinedDownloadTask from a DownloadTask.
  
  @param task the download task
  @param <T> the type of the DownloadTask
  @return the CombinedDownloadTask
  @throws NullPointerException if <code>task == null</code>