ConcatenatedByteSource(Iterable<? extends ByteSource> sources) {
      this.sources = checkNotNull(sources);
    }

    @Override
    public InputStream openStream() throws IOException {
      return new MultiInputStream(sources.iterator());
    }

    @Override
    public boolean isEmpty() throws IOException {
      for (ByteSource source : sources) {
        if (!source.isEmpty()) {
          return false;
        }