S-99-14


S-99-14 Duplicate the elements of a list.

Example:

 (dupli '(a b c c d)) 
 ; => (a a b b c c c c d d) 

Solution:

 (define dupli 
   (lambda (xs) 
     (if (null? xs) '() 
       (cons (car xs) 
             (cons (car xs) (dupli (cdr xs)))))))