static S nlMakeUnusedVar(S text) {
  L<S> tok = nlTok(text);
  new TreeSet<S> usedVars;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (t.startsWith("$") && l(t) > 1)
      usedVars.add(t);
  }
  for (int i = 0; i < 26; i++) {
    S var = "$" + (char) (((int) 'a') + i);
    if (!usedVars.contains(var))
      ret var;
  }
  throw fail("Help help help too many variables!");
}