;; double-all : list-of-number -> list-of-number
;; consumes a list of numbers and produces list of all numbers doubled
(define (double-all alon)
  (cond [(empty? alon) empty]
        [(cons? alon) (cons (* 2 (first alon))
                            (double-all (rest alon)))]))



  
;; extract-positives : list-of-number -> list-of-number
;; consumes a list of numbers and produces list of all positive numbers 
;; from list (consider 0 positive)
(define (extract-positives alon)
  (cond [(empty? alon) empty]
        [(cons? alon) (cond [(>= (first alon) 0) 
                             (cons (first alon) (extract-positives (rest alon)))]
                            [else (extract-positives (rest alon))])]))