Specify covariates that vary over different levels
of multi-membership grouping factors thus requiring
special treatment. This function is almost solely useful,
when called in combination with `mm`

.
Outside of multi-membership terms it will behave
very much like `cbind`

.

## Arguments

- ...
One or more terms containing covariates
corresponding to the grouping levels specified in `mm`

.

## Value

A matrix with covariates as columns.

## Examples

```
if (FALSE) {
# simulate some data
dat <- data.frame(
y = rnorm(100), x1 = rnorm(100), x2 = rnorm(100),
g1 = sample(1:10, 100, TRUE), g2 = sample(1:10, 100, TRUE)
)
# multi-membership model with level specific covariate values
dat$xc <- (dat$x1 + dat$x2) / 2
fit <- brm(y ~ xc + (1 + mmc(x1, x2) | mm(g1, g2)), data = dat)
summary(fit)
}
```