S-99-19

S-99-19 Rotate a list N places to the left.

Examples:

 (rotate '(a b c d e f g h) 3) 
 ; => (d e f g h a b c) 
  
 (rotate '(a b c d e f g h) -2) 
 ; => (g h a b c d e f) 

Hint: Use the predefined functions length and append, as well as the result of S-99-17.

Solution:

 (define (rotate xs n) 
   (if (< n 0) 
       (reverse (rotate (reverse xs) (- n))) 
       (let ((parts (split xs n))) 
         (append (cadr parts) (car parts)))))