#' @references
#'   Vehtari, A., Gelman, A., and Gabry, J. (2017). Practical
#'   Bayesian model evaluation using leave-one-out cross-validation and WAIC.
#'   \emph{Statistics and Computing}. 27(5), 1413--1432.
#'   doi:10.1007/s11222-016-9696-4. arXiv preprint:
#'   \url{https://arxiv.org/abs/1507.04544}
#'
#'   Yao, Y., Vehtari, A., Simpson, D., and Gelman, A. (2018) Using
#'   stacking to average Bayesian predictive distributions. \emph{Bayesian
#'   Analysis}, advance publication,  \doi{10.1214/17-BA1091}.
