(define head car)
(define tail cdr)

(define (accumulate op nv a b term next)
  (if (> a b)
      nv
      (op (term a)
(accumulate op nv (next a) b term next))))

(define (filter* p? lst)
  (foldr (lambda (el result) (if (p? el)
                                 (cons el result)
                                 result))
'() lst))

(
define
(foldr op nv l)
(
if
(null? l) nv
(op (car l) (foldr op nv (cdr l)))))

(
define
(foldl op nv l)
(
if
(null? l) nv
(foldl op (op nv (car l)) (cdr l))))