[plt-scheme] How to change the following code to make it handle nested lists?

From: SamuelXiao (foolsmart2005 at gmail.com)
Date: Mon Nov 24 01:50:00 EST 2008

I have following code:

(define (mysum list)
   (if (null? list)
      (+ (car list) (mysum (cdr list)))

It can handle the case summing up a list of numbers....however, I want
to handle case like '(1 (2 3) 4), how can I make it?
The code can handle (mysum '(1 2 3)) => 6, but when it comes to (mysum
'(1 (2 3) 4))=>error.  Any help would be appreciated.

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