Awesome great kata. Teaches implementing extention methods and argument null validation for iterators (yes, latter was totally not intuitive for me)

I would include this kata in all kinds of learning plans, instead of "look up algorithm on a random math site, rewrite it in c#, calculate fibonacci" stuff.

I spent days and days on this, nearly committed suicide, I was in despair, my entire life was destroyed.

But now that I've got it to run in 10 seconds: "Pffff, only 3 kyu?? This was too easy. Rate it 6, maximum 5!"