(define (accumulate op initial sequence) (if (null? sequence) initial (op (car sequence) (accumulate op initial (cdr sequence))))) (define (map p sequence) (accumulate (lambda (x y) (cons (p x) y)) '() sequence)) (define (append seq1 seq2) (accumulate cons seq2 seq1)) (define (length sequence) (accumulate (lambda (x n) (+ n 1)) 0 sequence))