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