Here's a shady looking Common Lisp macro. (defmacro f (n) (cond ((numberp n) n) ((not (listp n)) (error "not a number")) ((equal

Here's a shady looking Common Lisp macro. (defmacro f (n) (cond ((numberp n) n) ((not (listp n)) (error "not a number")) ((equal (car n) 'f) `(* -1 ,n)) (t n)))