Well, I can only guess too. :)

But the DB keeps track on which nodes I have voted on in one way or the other, so it shouldn't be impossible. The question is, is it stored on the node who has voted - easy and cheap query to count who has voted, or is it stored on the user where the votes have been cast - not as cheap a query (I wouldn't think at least). This query would need to be cheap, since it is potentially executed lots of times per rendered page.

Of course, another approach would be to add a field to the DB now, and start with this system from now on. If people want it. :)

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You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.