svoid retry(int attempts, Runnable r) {
  Throwable lastException = null;
  while ping (attempts-- > 0) {
    try {
      callF(r);
      ret;
    } catch print e {
      lastException = e;
    }
    print("Attempts remaining: " + attempts);
  }
  if (lastException != null)
    throw rethrow(lastException);
}