[Zope] Count specific objects

In subfolder "ref_links" I have a another folder called myLinksandURLs (which actually is a product called URL Directory). In that folder (myLinksandURLs) there are a couple of objects called Simple URL derived from the product called URL Directory. I now want to count the number of 'Simple URL' . I have tried this. <dtml-var "_.len('Simple URL')"> but that counts only the number of letters in Simple URL. How do I tell the _.len function to count how many objects exists in a specific folder?