// diff key: path, file sizes (or -1 if not existent)
static L<KeyValueDiff<S, Long>> diffZipFiles(File zip1, File zip2) {
  new L<KeyValueDiff<S, Long>> diffs;
  Map<S, Long> files1 = listZipAsNameToSizeMap(zip1);
  Map<S, Long> files2 = listZipAsNameToSizeMap(zip2);
  ret sorted(mapsToKeyValueDiffs(files1, files2));
}