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Exercise solution:
A counterpart to list-tail

Here are my functions:

(define (my-list-tail lst n)
  (if (= n 0)
      (my-list-tail (cdr lst) (- n 1))))

(define (list-prefix lst n)
  (if (= n 0)
      (cons (car lst) (list-prefix (cdr lst) (- n 1)))))

Notice that we must allocate memory to the result in list-prefix, whereas my-list-tail returns part of the original list (without allocating additional space).