public function current()
    {
        switch ($this->flags & 0x0F) {
            default:
            case self::CURRENT_ARRAY:
                return [$this->lhs->current(), $this->rhs->current()];
            case self::CURRENT_LHS:
                return $this->lhs->current();
            case self::CURRENT_RHS:
                return $this->rhs->current();
            case self::CURRENT_0:
                return null;
        }
    }