(define ints-from-to
  (lambda (lo hi)
    (if (> lo hi)
        '()
        (cons lo (ints-from-to (+ lo 1) hi)))))

(define length
  (lambda (lst)
    (if (null? lst)
        0
        (+ 1 (length (cdr lst))))))

(define sum
  (lambda (lst)
    (if (null? lst)
        0
        (+ (car lst) (sum (cdr lst))))))


(define filter
  (lambda (ok? lst)
    (cond ((null? lst)
           '())
          ((ok? (car lst))
           (cons (car lst) (filter ok? (cdr lst))))
          (else
           (filter ok? (cdr lst))))))