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5. Define a tail-recursive function digits->number that takes a list of digits and returns the number represented by those digits. For example, > (digits->number '17 6 1 5)) 7615 Any helper functions you ned should be defined within the body of digits->number using letrec. 6. Write a function, cond->if, which takes a cond expression, and transforms it into a set of nested if expressions. For example, > (cond->if '(cond (> x y) (- x y)) ((