(defclass data ()
 ((%total :accessor data-total
          :initform 0)
  (%count :accessor data-count
          :initform 0)))

(defun average (data)
  (/ (data-total data) (data-count data)))

(defun accumulate (data value)
  (incf (data-total data) value)
  (incf (data-count data)))