Oh, that's funny, thanks for pointing that out. I had exactly this in mind (or the proof that the set of rational numbers are countable), but mistakenly though

Oh, that's funny, thanks for pointing that out. I had exactly this in mind (or the proof that the set of rational numbers are countable), but mistakenly thought it was called the 'diagonal argument' because you would get the bijection to the natural numbers by counting diagonally through the table.