Website: indexterm, index?

I'm interesting in adding index entries to my WebSite sources and 
generating an index page. website-full.dtd appears to include docbook 
index elements in webpage.mix, but they don't validate, so I must be 
misunderstanding the dtd.

If there's no way to use docbook indexterms and index in WebSite, is 
there perhaps another way to do something similar?

Thanks,
Denis