local({
  previous_options <- options(warn = 2, digits = 8, stringsAsFactors = FALSE)
  on.exit(options(previous_options), add = TRUE)
  sum_to <- function(n) {
    if (!is.numeric(n) || length(n) != 1L || is.na(n) ||
        !is.finite(n) || n < 0 || n > 20 || n != floor(n)) {
      stop("Use one integer from 0 to 20")
    }
    if (n == 0) return(0)
    n + sum_to(n - 1)
  }
  cat("zero:", sum_to(0), "four:", sum_to(4), "\n")
  invisible(NULL)
})