static File makeDirNameUnique_startWithOne(File f, S ext) {
  File orig = f;
  int n = 1;
  S name = orig.getName();
  f = new File(dirOfFile(orig), name + ".1");
  while (f.exists())
    f = new File(dirOfFile(orig), name + "." + (++n));
  ret f;
}