(defmacro unless (test :rest body)
   `(if (not ,test) (progn ,@body)))
(defmacro assert (test)
   `(unless ,test
            (error "assert: value is false." ',test)))
(defun sum (sequence)
  (let ((res 0))
    (for ((xs sequence (cdr xs)))
      ((null xs) res)
      (setq res (+ res (car xs))))))
(defun average-of-absolutes (values)
  (the <list> values)
  ;; requires non-null list
  (assert (> (length values) 0))
  (let ((res (quotient (sum values) (length values))))
       ;; must ensure positive result
       (assert (>= res 0))
       (the <fixnum> res)))
;; (average-of-absolutes '(1 3))
;; (average-of-absolutes '())