static S formatDollarVars(S pat, SS vars) {
  L<S> tok = javaTok(pat);
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    S value = vars.get(t);
    if (value != null) tok.set(i, value);
  }
  ret join(tok);
}