*
   * {@snippet :
   * final ClosingFuture<BufferedReader> file1ReaderFuture = ...;
   * final ClosingFuture<BufferedReader> file2ReaderFuture = ...;
   * ListenableFuture<Integer> numberOfDifferentLines =
   *       ClosingFuture.whenAllSucceed(file1ReaderFuture, file2ReaderFuture)
   *           .call(
   *               (closer, peeker) -> {
   *                 BufferedReader file1Reader = peeker.getDone(file1ReaderFuture);