2.36¶

; The procedure accumulate-n is similar to accumulate except that it takes as its 
; third argument a sequence of sequences, which are all assumed to have the same 
; number of elements. It applies the designated accumulation procedure to combine 
; all the first elements of the sequences, all the second elements of the sequences, 
; and so on, and returns a sequence of the results. For instance, if s is a sequence 
; containing four sequences, ((1 2 3) (4 5 6) (7 8 9) (10 11 12)), then the value 
; of (accumulate-n + 0 s) should be the sequence (22 26 30). Fill in the missing 
; expressions in the following definition of accumulate-n:
; (define (accumulate-n op init seqs) 
;     (if (null? (car seqs))
;         nil
;         (cons (accumulate op init ⟨??⟩)
;               (accumulate-n op init ⟨??⟩))))

(define nil '())
(define (accumulate op initial sequence)
    (if (null? sequence) 
        initial
        (op (car sequence) (accumulate op initial (cdr sequence)))))

(define (accumulate-n op initial seqs)
    (if (null? (car seqs))
        nil
        (cons (accumulate op initial (map car seqs))
              (accumulate-n op initial (map cdr seqs)))))
(accumulate-n 
    + 
    0 
    (list (list 1 2 3) (list 4 5 6) (list 7 8 9) (list 10 11 12))) ; (22 26 30)