User:IssaRice/Belief propagation two coins and a bell example

This is an example that illustrates belief propagation in a simple polytree. The situation is given by Pearl (though he doesn't use it as an example of belief propagation). There are two coins, left and right, and a bell. The bell rings when the two coins land on the same side.

We can draw a Bayesian network like this: $L\to B\leftarrow R$, where $L$ is a random variable representing the left coin flip, $B$ the bell, and $R$ the right coin flip.