TEGAN: Will it tell us how to fly the Tardis?
NYSSA: I'm sure that's in here somewhere, once we find the index file.
TEGAN: How do we find the index file? Of course, if we had an index file, we could look it up in the index file, under index file. What am I saying? I'm talking nonsense.
NYSSA: Recursion isn't nonsense.
NYSSA: That's an example of recursion, when procedures fold back on themselves. If you had an index file, you could look it up in the index file.
TEGAN: If. My Dad used to say that if was the most powerful word in the English language.
NYSSA: Recursion’s a powerful mathematical concept, but I don't see how it can help us now.
TEGAN: If. I F! Stands for index file!