nspl\ds\Set::difference PHP Метод

    public function difference($sequence)
    {
        if ($sequence instanceof Set) {
            $result = new Set();
            $result->array = array_diff_key($this->array, $sequence->array);
            return $result;
        }
        args\expects(args\traversable, $sequence);
        $result = new Set();
        $intersection = $this->intersection($sequence);
        foreach ($this->array as $element) {
            $elementKey = static::getElementKey($element);
            if (!isset($intersection->array[$elementKey])) {
                $result->array[$elementKey] = $element;
            }
        }
        return $result;
    }