I would like to express a model with multiple feature tested, and herarchical relationships and contrains across parameters.

```
y1 ~ 1
y2 ~ 1
y3 ~ 1
sigma_1,2,3 ~ Intercept_1,2,3 * b # or more specifically -> log(sigma_1,2,3) ~ inv_softmax(Intercept_1,2,3) * b
[Intercept_1,2,3] = simplex
```

Is this possible in brms?