code: @Override
    public byte[] call(final byte[] memento) throws IOException {
      try (final DataOutputStream outputStream = outputStreamProvider.create("hello")) {
        outputStream.writeBytes("Hello REEF!");
      }
      return null;
    }
docs: Receives an output stream from the output service and writes "Hello REEF!" on it.
  
  @param memento the memento objected passed down by the driver.
  @return null
  @throws java.io.IOException