code: static void mergeInto(List<Path> parts, OutputStream out)
        throws IOException {
      for (final Path part : parts) {
        Files.copy(part, out);
        Files.delete(part);
      }
    }
docs: Merge the given part files in order into an output stream.
  This deletes the parts.
  @param parts the part files to merge
  @param out the stream to write each file into, in order
  @throws IOException