static L<File> conceptBackupFiles(S progID) {
  Pattern pat = Pattern.compile("^(.*)\\.backup(20\\d\\d)(\\d\\d)(\\d\\d)-(\\d\\d)$");
  File dir = programDir(progID);
  new L<File> l;
  for (File f : listFilesNotDirs(dir, newFile(dir, "backups"))) {
    S s = f.getName();
    Matcher matcher = pat.matcher(s);
    continue unless matcher.find();
    S originalName = matcher.group(1);
    continue unless eq(originalName, "concepts.structure.gz");
    l.add(f);
  }
  ret l;
}