This is a good explanation of UDT, its relationship to EDT, and why UDT solves the Smoking Lesion Problem when EDT doesn't. The Smoking Lesion Problem is actually a central motivation for UDT. (The story is that I was reading about the Smoking Lesion Problem, and thought something like "EDT would solve this correctly if the agent knew its own source code and conditioned on that first before deciding its action", which doesn't quite work, but helped me to later formulate UDT). I've been lazy about writing a post on this, so I'm glad Paul has figured it out and Randaly has put it in written form. (None of the previous discussions of the Smoking Lesion Problem I've seen on LW seems to have really "gotten it".)