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Testing for List Membership (Ex. 7.8a)

Desired behavior of procedure element?: 
          (element? 8 (integers-from-to 1 15)) ⇒ #t

          (element? 20 (integers-from-to 1 15)) ⇒ #f
Q: When is an item an element of a list?
(Hint: Answer in terms of the head and the tail.)
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