Exercise 7.8a, p. 174

     (element-of? 'foo '(1 3 5 foo 9)) ⇒ #t

     (element-of? 'bar '(1 3 5 foo 9)) ⇒ #f

• An item cannot be an element of _____

• If a list is not empty, an item is an element if _____

• Otherwise, an item is an element if _____