I have a nx7 matrix in stan. I need to access the 1st and 7th column if a condition is met, but access the 2nd-7th columns under another condition. I am basically running a regression model like a seemingly unrelated regression but some of the terms are irrelevant for the sub regressions. In R I would do this with mat[,c(1,7)] but how would I do that in stan?

```
data {
matrix[m,n] M;
}
```