static File countTillNewFileOrKeep(File base) {
  ret countTillNewFileOrKeep(base, "");
}

static File countTillNewFileOrKeep(File base, S suffix) {
  File f = newFile(f2s(base) + suffix);
  long i = 1;
  while (f.exists())
    f = newFile(f2s(base) + ++i + suffix);
  ret f;
}