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