code: @Override
    public R finish(Collection<? extends OutputWriter<O>> writers) throws IOException {
      ImmutableList.Builder<OutputWriter<ByteBuffer>> sinkWriters = ImmutableList.builder();
      for (OutputWriter<O> w : writers) {
        Writer<O> writer = (Writer<O>) w;
        sinkWriters.add(writer.out);
      }
      return sink.finish(sinkWriters.build());
    }
docs: Returns whatever the underlying {@code Output}'s {@link #finish} method returns.