I was wondering how to obtain the posterior prediction based on a grouping variable from `stan_glm()`

?

For example, if I have a binary `(0, 1)`

coded grouping variable called `"vs"`

in my data, how can I obtain the prediction for `vs == 0`

and `vs == 1`

?

Here is my `R`

code:

```
fit <- stan_glm(mpg ~ ., data = mtcars)
posterior_predict(fit, newdata = ??)
```