static S hijackPrint_tee(Runnable r) {
  final new StringBuilder buf;
  O old = interceptPrintInThisThread(new F1<S, Bool>() {
    Bool get(S s) {
      buf.append(s);
      true;
    }
  });
  try {
    callF(r);
    ret str(buf);
  } finally {
    interceptPrintInThisThread(old);
  }
}