public function installedData(string $slug, string $key): string
    {
        if (!$slug || !$key) { // Either empty?
            return ''; // Not possible.
        }
        switch ($key) {
            case 'basename':
                $key = 'Basename';
                break;
            case 'name':
                $key = 'Name';
                break;
            case 'version':
                $key = 'Version';
                break;
            case 'description':
                $key = 'Description';
                break;
            case 'url':
                $key = 'PluginURI';
                break;
            case 'text_domain':
                $key = 'TextDomain';
                break;
            case 'author':
                $key = 'Author';
                break;
            case 'author_url':
                $key = 'AuthorURI';
                break;
            default: // Default case.
                return ''; // Unknown key.
        }
        $installed_plugins = $this->s::allInstalledPlugins(); // « Cached statically.
        return !empty($installed_plugins[$slug][$key]) ? (string) $installed_plugins[$slug][$key] : '';
    }