Person i is asked to choose among options C_i, which varies in size across people. Our model includes the softmax:

\frac{\exp(\eta_{ic'})}{\sum_{c \in C_i} \exp(\eta_{ic})}

What is the best way to code this? How should we store the C_i? Stan doesn’t have ragged arrays, but perhaps as vectors of 0/1s of length equal to the total number of options?

```
exp(eta[ii[n],cc[n]])/sum(exp(eta[ii[n],C_i]))
```