public function key()
    {
        switch ($this->flags & 0xF0) {
            default:
            case self::CURRENT_ARRAY:
                return [$this->lhs->key(), $this->rhs->key()];
            case self::CURRENT_LHS:
                return $this->lhs->key();
            case self::CURRENT_RHS:
                return $this->rhs->key();
            case self::CURRENT_0:
                return null;
        }
    }