static S renameDollarVarsTo123(S s) {
  ret renameDollarVarsTo123(javaTok(s));
}

static S renameDollarVarsTo123(LS tok) {
  new SS map;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (isDollarVar(t)) {
      S v = map.get(t);
      if (v == null)
        map.put(t, v = "$" + (l(map)+1));
      tok.set(i, v);
    }
  }
  ret join(tok);
}

Began life as a copy of #1010854