Pinq\Traversable::minimum PHP Method

    public function minimum(callable $function = null)
    {
        $min = null;
        $first = true;
        foreach ($this->mapIterator($function) as $value) {
            if ($value < $min || $first) {
                $min = $value;
                $first = false;
            }
        }
        return $min;
    }