sS lambda0Like hijackPrint(Runnable r) {
  new StringBuilder buf;
  O old = interceptPrintInThisThread(new F1<S, Bool>() {
    public Bool get(S s) {
      buf.append(s);
      false;
    }
  });
  try {
    r?.run();
    ret str(buf);
  } finally {
    interceptPrintInThisThread(old);
  }
}