Exercise 5.19

(define (insert-and sent)
  (se (bl sent) 'and (last sent)))

(define (middle-names name)
  (bf (bl name)))

(define (query sent)
  (se (second sent)
      (first sent)
      (bf (bf (bl sent)))
      (word (last sent) '?)))

(define (second sent)
  (first (bf sent)))

(define (sort2 sent)
  (if (<= (first sent) (second sent))
      sent
      (se (second sent) (first sent))))

(define (second sent)
  (first (bf sent)))

(define (valid-date? m d y)
  (cond ((< y 0) #f)
	((has-31-days? m) (within? d 1 31))
	((has-30-days? m) (within? d 1 30))
	((= 2 m) (if (leap-year? y)
		     (within? d 1 29)
		     (within? d 1 28)))
	(else #f)))

(define (has-31-days? m)
  (member? m '(1 3 5 7 8 10 12)))

(define (has-30-days? m)
  (member? m '(4 6 9 11)))

(define (within? num low high)
  (and (<= num high)
       (>= num low)))

(define (leap-year? y)
  (if (divisible? y 100)
      (divisible? y 400)
      (divisible? y 4)))

(define (divisible? num divider)
  (= 0 (remainder num divider)))

(define (plural wd)
  (cond ((ends-in-vowel-and-y? wd) (word wd 's))
	((ends-in? wd 'y) (word (bl wd) 'ies))
	((ends-in? wd 'x) (word wd 'es))
	(else (word wd 's))))

(define (ends-in? wd letter)
  (equal? (last wd) letter))

(define (ends-in-vowel-and-y? wd)
  (and (ends-in? wd 'y)
       (vowel? (last (bl wd)))))

(define (vowel? letter)
  (member? letter '(a e i o u)))

(define (greet name)
  (cond ((equal? name '(stanley livingston)) '(dr livingston i presume?))
	((equal? (first name) 'dr) (if (member? (last name) '(jr sr))
				       (se '(hello dr) (last (bl name)))
				       (se '(hello dr) (last name))))
	((member? (first name) '(queen king)) (se '(hello your majesty)))
	(else (se 'hello (first name)))))

(define (describe-time secs)
  (cond ((> secs 86400) (se (exact->inexact (/ secs 86400)) 'days))
	((= secs 86400) '(one day))
	((> secs 3600) (se (exact->inexact (/ secs 3600)) 'hours))
	((= secs 3600) '(one hour))
	((> secs 60) (se (exact->inexact (/ secs 60)) 'minutes))
	((= secs 60) '(one minute))
	((> secs 1) (se secs 'seconds))
	((= secs 1) '(one second))
	(else 'huh?)))