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waterloo wrote (on Sun, 5 Jul 2009 at 10:13 +0800):

> yes,it is clearer . thanks
>
> In 11.1.2 An Example:`print-elements-of-list', I can not understand why
> there is a `nil' in the context .

This is explained at the end of the section:

 Since the last expression in the function is the `while' loop, and
 since `while' loops always return `nil', a `nil' is printed after the
 last element of the list.