// sorted by descending date
static L<File> conceptBackupFiles(S progID) {
  ret conceptBackupFiles(programDir(progID));
}

static L<File> conceptBackupFiles(File dir) {
  S regexp = "^(.*)\\.backup(20\\d\\d)(\\d\\d)(\\d\\d)-(\\d+)$";
  new L<File> l;
  for (File f : listFilesNotDirs(dir, newFile(dir, "backups"))) {
    if (isEmpty(f)) continue;
    S s = f.getName();
    S originalName = regexpFirstGroup(regexp, s);
    continue unless eq(originalName, "concepts.structure.gz");
    l.add(f);
  }
  ret sortFilesByNameDesc(l);
}