#' @param re_formula formula containing group-level effects to be considered in the prediction.
#' If `NULL` (default), include all group-level effects; if `NA`, include no group-level effects.
#' Some model types (such as [brms::brmsfit] and [rstanarm::stanreg-objects]) allow
#' marginalizing over grouping factors by specifying new levels of a factor in `newdata`. In the case of
#' [brms::brm()], you must also pass `allow_new_levels = TRUE` here to include new levels (see
#' [brms::posterior_predict()]).
