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A "Natural number" (natural) in this assignment means the type defined in the text. Your file should include a data definition for natural. Unless the problem statement stipulates otherwise, the only built-in operations you may use with values of this type or variants (such as natural>=1) are the constructors, accessors, and recognizers for the type and the equal? (or =}) operation. For natural , the constructor is add1 ; the accessor is sub1 , and he recognizers are zero? and positive? .
For variants, the constructor is typically add1 (unless the variant consists of multiples of m for m>1 such as the even numbers for which the constructor is add2 (+ ... m)), the accessor is typically sub1 (subk (- ... m) if the constructor is addk (+ ... m)), and the recognizers are typically (equal? ... k) , where k is the base number, (zero? for natural), and (> ... k) (positive? for natural).
Problems from the book (HTDP) with some customization
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