public function getResource($resource) : ?string
    {
        Preconditions::checkState($this->isPsr0Compatible(), "Class '%s' must be PSR-0 compatible!", $this->getName());
        $slashedFileName = $this->getSlashedFileName();
        $filePath = $resource[0] == '/'
            ? str_replace("/{$this->getSlashedName()}.php", '', $slashedFileName) . $resource
            : dirname($slashedFileName) . '/' . $resource;
        return is_file($filePath) ? $filePath : null;
    }