static Collection<S> ai_powerWordsExpansions(ParsedPowerWords ppw, S... words) {
  if (ppw == null) null;
  Set<S> seen = ciSet();
  new LS out;
  LPair<LS> equations = ai_parsedPowerWordEquations(ppw);
  while licensed {
    bool change = false;
    fOr (ParsedPowerWords.Equation eq : equations) {
      if (containsAll(seen, eq.lhs))
        if (addAllNotContainedIn(out, eq.rhs, seen))
          set change;
    }
    if (!change) break;
  }
  ret out;
}