variable XT jointly with all evidence at all times now one thing that might come up in practice is that as you run this the way it's shown here even though mathematically is the simplest way you might run into numerical problems because actually compute a joint over more and more variables the actual probability value will keep going down and down and you might get under flow where you get numbers that are below numbers you can represent in floating point so in practice even though the math is kind of simple and cleanest when