<a href="http:&#x2F;&#x2F;intellectualicebergs.org&#x2F;kidputer&#x2F;" rel="nofollow">http:&#x2F;&#x2F;intellectualicebergs.org&#x2F;kidputer&#x2F;</a> was my original source.<p>I’ve tweaked the exercise a bit though, so I’ll see if I can get those changes written up - the original author gave us permission to use the work as we saw fit back when I worked at the Dept of Comp.Sci. in Oxford.