delete-car function

I stumbled on this page and started to solve the problems, Here is solution to the ex4.

Define (delete-car list) to modify and return list with the first element of list deleted.

> (setq l (list 'a 'b 'c))
(A B C)
> (delete-car l)
(B C)
> L
(B C)

Solution:

(defun delete-car (l)
 (setf (car l) (cadr l))
 (let ((tmp (cddr l)))
   (setf (cdr l) tmp))
 (if (equal l '(())) nil l))