Pinq\Traversable::aggregate PHP Method

    public function aggregate(callable $function)
    {
        $hasValue = false;
        $aggregateValue = null;
        foreach ($this->elements as $value) {
            if (!$hasValue) {
                $aggregateValue = $value;
                $hasValue = true;
                continue;
            }
            $aggregateValue = $function($aggregateValue, $value);
        }
        return $aggregateValue;
    }