sS dollarVarsToFixedString(S replacement, S s, L<S> varNames_out) {
  L<S> tok = javaTok(s);
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (isDollarVar(t)) {
      listAdd(varNames_out, t);
      tok.set(i, replacement);
    }
  }
  ret join(tok);
}

Began life as a copy of #1010854