;;; Procedure:
;;;   assoc
;;; Parameters:
;;;   key, a Scheme value
;;;   alist, an association list
;;; Purpose:
;;;   Find an entry with key key in alist.
;;; Produces:
;;;   entry, a Scheme value
;;; Preconditions:
;;;   No additional
;;; Postconditions:
;;;   If there is an index, i, such that
;;;     (equal? key (car (list-ref alist i)))
;;;   then entry is the first such entry
;;;   Otherwise, entry is false (#f)
(define assoc
  (lambda (key alist)
    (cond
       ; If there are no entries left in the association list,
       ; there are no entries with the given key.
       ((null? alist) 
        #f)
       ; If the key we're looking for is the key of the first
       ; entry, then use that entry.
       ((equal? key (car (car alist))) 
        (car alist))
       ; Otherwise, look in the rest of the association list.
       (else 
        (assoc key (cdr alist))))))