We should very carefully document how indexes are used in this model and how they map back to the notation in the paper. We should then use them very consistently. They may already be consistent, but it is a little hard to tell because I don't (intuitively) know what all of them mean. We have indexes for rating values, betas, thresholds, etc., and we need to know, when we call for example the derivative function, what all the indices mean.