thickness in a branching l-system

I spent a surprisingly large part of the day debugging the addition of thickness to l-systems in the attempt to make a nice-looking tree. I finally did it, though it still has some shortcomings. I was surprised how little help AI was in this. It consistently sent me in the wrong direction. In the end I had to rely entirely on myself. I guess there were few examples online of how to do this... perhaps none. This points to an interesting limitation of current AIs.

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