public function redo()
    {
        end($this->history);
        for ($i=0; $i<$this->history_offset-1; $i++) {
            prev($this->history);
        }
        $index = current($this->history);
        if (!empty($index)) {
            $this->current_passage = $index;
            $this->history_offset--;
            $this->storyline[] = $this->current_passage;
        }
        return $this->getCurrentPassage();
    }