(defun rank (l &optional (n 0) (h (make-hash-table)))
  (labels ((pop-repeats (b4 l sum) 
	     (cond ((null l)         n)
		   ((eql b4 (car l)) (incf n) 
		                     (pop-repeats b4 (cdr l) (+ sum n))
		    (t               (/ sum n))))))
    (if (null l)
      h
      (let ((head (pop l)))
	(setf (gethash head h) 
	      (pop-repeats head l n))
	(rank l (1+ n) h)))))