public function push($value)
    {
        if (!$this->current->value) {
            $this->current->value = $value;
        } else {
            $temp = $this->current;
            $this->current = new ClassNode($value, $temp->prev);
            $this->current->next = $temp;
            $temp->prev = $this->current;
            if ($temp === $this->head) {
                $this->head = $this->current;
            } else {
                $this->current->prev->next = $this->current;
            }
        }
    }