(define make-monitored
  (lambda (f)
    (let ((counter 0))
      (lambda (message)
	(cond ((eq? message 'reset-count) (set! counter 0) 0)
	      ((eq? message 'how-many-calls?) counter)
	      (else (set! counter (1+ counter))(f message)))))))

(define s (make-monitored sqrt))

(s 2)
(s 'how-many-calls?)
(s 2)
(s 'how-many-calls?)
(s 'reset-count)
(s 'how-many-calls?)