static F0<S> abcDollarVarMakerWithoutTaken(final Set<S> taken) {
  final ItIt<S> it = filterIterator(allWordsOfLowerCaseAlphabet(),
    func(S s) -> bool { !contains(taken, "$" + s) });
  it.next(); // skip "$"
  ret func -> S { "$" + it.next() };
}