3 (define (make-monitored f)
7 ((eq? x 'how-many-calls?) num-calls)
10 (set! num-calls (+ num-calls 1))
14 > (define (square x) (* x x))
17 > (define mf (make-monitored square))
18 > (mf 'how-many-calls?)
22 > (mf 'how-many-calls?)
26 > (mf 'how-many-calls?)