About Repeated Expansion

[Xue Fuqiao]

The GNU Emacs Lisp 
manual(http://www.gnu.org/software/emacs/manual/html_node/elisp/Repeated-Expansion.html#Repeated-Expansion)
 says:

 Here is an example of how such side effects can get you into trouble:
     (defmacro empty-object ()
       (list 'quote (cons nil nil)))
     (defun initialize (condition)
       (let ((object (empty-object)))
         (if condition
             (setcar object condition))
         object))
If initialize is interpreted, a new list (nil) is constructed each time 
initialize is called. Thus, no side effect survives between calls. If 
initialize is compiled, then the macro empty-object is expanded during 
compilation, producing a single “constant” (nil) that is reused and altered 
each time initialize is called. 

It says if `initialize' is compiled, the (nil) will be reused and altered each 
time `initialize' is called.  But there is no expression that alters 
`empty-object', why will it be altered?
-- 
Best regards.