// newest first
static L<File> sortFilesByDateDesc(Collection<File> files) {
  ret sortedByComparator(files, new Comparator<File>(){
    public int compare(File b, File a) {
      long x = a.lastModified(), y = b.lastModified();
      return x < y ? -1 : x == y ? 0 : 1;
    }
  });
}