public function collection($label)
    {
        if (is_string($label) === false) {
            throw new \InvalidArgumentException(
                'Collection icon label must be a string.'
            );
        }

        if (isset($this->collection[$label]) === false) {
            throw new CollectionIconException(
                'Collection icon "' . $label . '" does not exist.'
            );
        }

        return $this->collection[$label];
    }