sS arbitraryVarsToStars(S s, Set<S> vars, LS varNames_out) {
  if (empty(vars)) ret s;
  L<S> tok = javaTok(s);
  bool change;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (vars.contains(t)) {
      listAdd(varNames_out, t);
      tok.set(i, "*");
      set change;
    }
  }
  ret change ? join(tok) : s;
}