private function AddSitemapChangeFrequencyField()
    {
        $name = 'SitemapChangeFrequency';
        $value = $this->page->Exists() ? $this->page->GetSitemapChangeFrequency() :
            (string)ChangeFrequency::Weekly();
        
        $field = new Select($name, $value);
        $values = ChangeFrequency::AllowedValues();
        foreach ($values as $val)
        {
            $field->AddOption($val, Trans('Core.Sitemap.ChangeFrequency.' . ucfirst($val)));
        }
        $this->AddField($field);   
    }