public function unique()
    {
        $count = count($this->sfa);
        $unique = new SplFixedArray($count);
        $newCount = 0;
        $lastUnique = null;

        foreach ($this->sfa as $el) {
            for($i = $newCount; $i >= 0; $i--) {
                // going from back to forward to find sequence of duplicates faster
                if ($unique[$i] === $el) {
                    continue 2;
                }
            }
            $unique[$newCount++] = $el;
        }

        $unique->setSize($newCount);
        return new static($unique);
    }