version 2 pré-béta

Compte tenu des commentaires ci-dessus, voici une version un peu moins approximative du bousin

(setq t0 nil)
(setq array (list))
(defun sum(list) 
 (if (null list)
 0
 (+ 
 (nth 0 list) 
 (sum (rest list))
 ) 
 ) 
 )
(defun mean(list)
 (/ (sum list) (length list))
 )
(defun tap ()
 "Record a tap"
 (interactive)
 (setq t1 (float-time()))
 (if t0 (push (- t1 t0) array))
 (if (> (length array) 9)
 (progn
 (message "BPM %S" (* 60 (/ 1. (mean array))))
 (setq array (reverse (cdr (reverse array))))
 ))
 (setq t0 t1))
(global-set-key "\C-x\C-s" 'tap)