; adds all values passed as parameters (var-args).
(define sum-all (lambda L (foldl + 0 L)))

; returns list of min -- max.  Like -- from ML.
(define (range min max)
  (if (> min max) null
      (cons min (range (+ 1 min) max))))

; our implementation of higher-order functions map and filter.
(define (mapx f lst)
  (if (null? lst) ()
      (cons (f (car lst)) (mapx f (cdr lst)))))

; our implementation of higher-order functions map and filter.
(define (filterx p lst)
  (cond ((null? lst) ())
        ((p (car lst)) (cons (car lst) (filterx p (cdr lst))))
        (else (filterx p (cdr lst)))))