static LPair<S> hijackPrintWithStackTraces(O r) {
  new LPair<S> out;
  temp tempInterceptPrint(new F1<S, Bool>() {
    Bool get(S s) {
      if (nempty(s))
      out.add(pair(s, getStackTrace()));
      false;
    }
  });
  callF(r);
  ret out;
}