Common Lisp isn't a "purely functional" language, it supports every paradigm. It allows silly things like... (let ((pair (cons 1 nil))) (setf (cdr pair)

Common Lisp isn't a "purely functional" language, it supports every paradigm. It allows silly things like... (let ((pair (cons 1 nil))) (setf (cdr pair) pair) (list (first pair) (second pair) (third pair))) ;; => (1 1 1)