}

    @Override
    public String toString() {
      return "ByteSource.empty()";
    }
  }

  private static final class ConcatenatedByteSource extends ByteSource {

    final Iterable<? extends ByteSource> sources;

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

    @Override