[Zope] Count specific objects

[peter]

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?