implementing time values, again
(define (mk-time hh mm)
(if (< mm 0)
(mk-time (- hh 1) (+ 60 mm))
(let ((extra-hh (quotient mm 60))
(new-mm (remainder mm 60)))
(cons (+ hh extra-hh) new-mm))))
(define (+time t1 t2)(mk-time (+ (hours t1) (hours t2)) (+ (mins t1) (mins t2))))
(define (-time t1 t2)(mk-time (- (hours t1) (hours t2)) (- (mins t1) (mins t2))))
(define (*time t k) (mk-time (* k (hours t)) (* k (mins t))))
(define (hours t) (car t))
(define (mins t) (cdr t))