Order Attribute

Hello!

I build the navigation bar of our website by iterating over the Silva  
Publications in the Silva Root.

<span tal:define="items python:root.silva.objectValues('Silva  
Publication')">

What I would like to do is order this list according to the ordering  
you see in the Silva Management Screen, so that if you move something  
up and down there this is reflected in our navigation bar. This could  
look something like this:

<span tal:define="items python:root.silva.objectValues('Silva  
Publication')"
             sorted_items python:sequence.sort( items,  
(('_____','cmp'),) )">

My Problem is that I can't find out how to get at the information  
which position was assigned to a Silva Publication. I looked through  
all of the service_metadata, but I can't find anything.

It would be very kind if you could give me a quick hint.

Thanks

Timo