*
       * https://github.com/typetools/checker-framework/issues/3030
       *
       * https://github.com/typetools/checker-framework/issues/3236
       */
      String key = (String) requireNonNull(e.nextElement());
      /*
       * requireNonNull is safe because the key came from propertyNames...
       *
       * ...except that it's possible for users to insert a string key with a non-string value, and

     * properly update its state.
     */
    void promoteToNext(E e) {
      if (nextElements.remove(e)) {
        nextElements.push(e);
      } else {
        throw new UnknownElementException(nextElements, e);
      }
    }

    private E transferElement(Stack<E> source, Stack<E> destination) {
      if (source.isEmpty()) {

     * properly update its state.
     */
    void promoteToNext(E e) {
      if (nextElements.remove(e)) {
        nextElements.push(e);
      } else {
        throw new UnknownElementException(nextElements, e);
      }
    }

    private E transferElement(Stack<E> source, Stack<E> destination) {
      if (source.isEmpty()) {