exam9.5.2

;; mht created on Nov 15, 2011

;; data definition:
;; A list-of-symbols is either
;; 1. the empty list, empty, or
;; 2. (cons s los) where s is a symbol and los is a list of symbols.

;; how-many-symbols : list-of-symbols -> numbers
;; to compute the number of items in a-list-of-symbols
;; examples: (how-many-symbols (cons 'a (cons 'b empty))) -> 2
(define (how-many-symbols a-list-of-symbols)
  (cond
    [(empty? a-list-of-symbols) 0]
    [else
     (+
      (how-many-symbols (rest a-list-of-symbols))
      1)]))

;; test
(= (how-many-symbols (cons 'a (cons 'b empty)))
   2)
(= (how-many-symbols empty)
   0)