static Collection<S> ai_powerWordsExpand(ParsedPowerWords ppw, Collection<S> words) {
  if (ppw == null) null;
  Set<S> seen = cloneCISet(words);
  new LS out;
  LPair<LS> equations = ai_parsedPowerWordEquations(ppw);
  while licensed {
    bool change = false;
    fOr (Pair<LS> eq : equations) {
      if (containsAll(seen, eq.a))
        for (S s : eq.b)
          if (seen.add(s)) {
            out.add(s);
            set change;
          }
    }
    if (!change) break;
  }
  ret out;
}