@Override
    @ParametricNullness
    public T next() {
      if (valueOrSentinel == SENTINEL) {
        throw new NoSuchElementException();
      }
      // The field held either a T or SENTINEL, and it turned out not to be SENTINEL.
      @SuppressWarnings("unchecked")
      T t = (T) valueOrSentinel;
      valueOrSentinel = SENTINEL;
      return t;
    }
  }

  /**