Thanks David. That does just what I wanted. Now generate-id finally makes more sense to me. I got rid of
all the // in my stylesheet, since your example showed me well how to use the keys I had set up.

The only thing I don't understand is why you don't use a [1] in order to get the first member of the key:

[generate-id()=generate-id(key('topics',.)[1])] instead of:
[generate-id()=generate-id(key('topics',.))]

I mean, it works how you have it, but I don't get why you don't need to use it.
What exactly is your expression doing?