[plt-scheme] multiply-by-pi HTDP 11.5.2

plt-scheme-request at list.cs.brown.edu wrote:
> http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-15.html#node_sec_11.5
> 
> I can only solve this problem by using + for  the recursion. because my add 
> function only works when at least one of the inputs is a natural number - 

That's okay. The question doesn't ask you to eliminate + from 
multiply-by-pi, only from multiply. --PR