I would like to be able to constrain portions of the covariance matrix of group effects using a hyperparameter. I don’t think that the solution to this problem is the `gr`

function because I’m not trying to represent the covariance structure among the group units, but rather trying to create a custom group-effects covariance matrix sort of like `pdClass`

does for `lem4`

models.

The specific covariance structure I’m interested in has to do with the time-ordered nature of one of the random effects.

If the data are generated

```
set.seed(12)
Sigma <- matrix(c(
1, .5, .25, .125, .0625,
.5, 1, .5, .25, .125,
.25, .5, 1, .5, .25,
.125, .25, .5, 1, .5,
.0625, .125, .25, .5, 1), ncol = 5)
rep_means <- MASS::mvrnorm(1000, mu = rep(0, 5), Sigma = Sigma)
rep_means_contrast <- cbind(rep_means[, 1], apply(rep_means[, 2:5], 2, function(x){
x - rep_means[, 1]
}))
d <- do.call(rbind, lapply(1:dim(rep_means)[[1]], function(id){
arow <- rep_means[id, ]
y1 <- rnorm(length(arow), mean = arow, sd = 1)
y2 <- rnorm(length(arow), mean = arow, sd = 1)
return(data.frame(id = id,
rep = as.factor(rep(1:length(arow), 2)),
y = c(y1, y2)))
}))
```

and if the model is

```
f <- bf( y ~ 0 + rep + (0 + rep | id) )
```

I’d like to capture a pooled estimate of the off-diagonals of the group effect of rep using a hyperparameter, as well as provide some prior information that might help speed up sampling. Or in other words, I’d like the model to reflect the possibility that the data generating process is essentially AR(1).

Is there any way to do this in `brms`

? I could imagine altering the model code produced to create a transformed parameter that is the covariance matrix and placing hyperparameters on the cells of that matrix, but it seems like there might be a better way.