As posteriors are priors that have been updated after observing some data,
the goal of this function is to un-update the posteriors to obtain models
representing the priors. These models can then be used to examine the prior
predictive distribution, or to compare priors with posteriors.

This function in used internally to compute Bayes factors.

## Usage

```
unupdate(model, verbose = TRUE, ...)
# S3 method for stanreg
unupdate(model, verbose = TRUE, ...)
# S3 method for brmsfit
unupdate(model, verbose = TRUE, ...)
# S3 method for brmsfit_multiple
unupdate(model, verbose = TRUE, newdata = NULL, ...)
# S3 method for blavaan
unupdate(model, verbose = TRUE, ...)
```