// newest first
static L<File> sortFilesByDateDesc(Collection<File> files) {
  ret sortedByComparator(files, new Comparator<File>{
    AutoMap<File, Long> dates = new AutoMap<File, Long>(lambda1 lastModified); // cache to ensure comparator is sane
    
    public int compare(File b, File a) {
      long x = dates.get(a), y = dates.get(b);
      return x < y ? -1 : x == y ? 0 : 1;
    }
  });
}