SICP по-русски

В этом упражнении нет каких-то подводных камней. Все достаточно прямолинейно и демонстрирует концепцию барьеров абстракций.

(define (make-segment start-point end-point)
  (cons start-point end-point))

(define (start-point segment)
  (car segment))

(define (end-point segment)
  (cdr segment))

(define (make-point x y)
  (cons x y))

(define (x-point point)
  (car point))

(define (y-point point)
  (cdr point))

(define (average a b)
  (/ (+ a b) 2.0))

(define (midpoint-segment segment)
  (make-point (average (x-point (start-point segment))
                       (x-point (end-point segment)))
              (average (y-point (start-point segment))
                       (y-point (end-point segment)))))

Вот и всё.