;;; ***************************************************
;;; procedure sum-n takes integer n >= 0 as input and
;;; returns the sum of the numbers up to and including n.
;;;

(define (sum-n n)
   (cond ((zero? n) 0)
         (else (+ n (sum-n (- n 1))))
   )
)

;;; ***************************************************
;;; procedure length takes a list as input and returns
;;; its length.
;;;

(define (length x)
   (cond ((null? x) 0)
         (else (+ 1 (length (cdr x))))
   )
)

;;; ***************************************************
;;; procedure abs-list takes a list of numbers as input
;;; and returns a list containing the absolute value of each
;;; number in the list.
;;; (Note this is not an efficient implementation.  Why?)

(define (abs-list l1)
    (cond ((null? l1) '())
    (else (cons 
      (if (> 0 (car l1)) (- (car l1)) (car l1))
      (abs-list (cdr l1))))
    )
)

;;; ***************************************************
;;; procedure append takes two lists as input and returns
;;; one list that is the second list appended to the first
;;;

(define (append l1 l2)
  (if (null? l1) l2
    (cons (car l1) (append (cdr l1) l2))
  )
)