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#lang racket
(provide accumulate
accumulate-n
fold-right
fold-left)
(define (append list1 list2)
(if (null? list1)
list2
(cons (car list1) (append (cdr list1) list2))))
(define (length items)
(define (length-iter a c)
(if (null? a)
c
(length-iter (cdr a) (+ 1 c))))
(length-iter items 0))
(define (list-ref items n)
(if (= n 0)
(car items)
(list-ref
(cdr items)
(- 1 n))))
(define (last-pair l)
(if (null? (cdr l))
l
(last-pair (cdr l))))
(define (accumulate op initial sequence)
(cond ((null? sequence) initial)
(else (op (car sequence)
(accumulate op initial (cdr sequence))))))
(define (accumulate-n op init seqs)
(if (null? (car seqs))
'()
(cons (accumulate op init (map car seqs))
(accumulate-n op init (map cdr seqs)))))
(define (fold-right op initial sequence)
(accumulate op initial sequence))
(define (fold-left op initial sequence)
(define (iter result rest)
(if (null? rest)
result
(iter (op result (car rest))
(cdr rest))))
(iter initial sequence))
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