[racket] slides unit testing

Hi,

I remember that someone, some time ago (how precise!), posted on this list a link to a couple of slides that nicely and concisely illustrate the advantages of pure functions vs. impure ones when doing testing. 

I wasn't able to retrieve the post from Google or my inbox. If someone remembers---at least its author should ;))--can you please send me the link?

Thanks in advance!

-- Éric