There's something I'd like to address about "Index File". From Castrovalva:
- TEGAN: Tardis Information System. Ready For Entry.
- NYSSA: A databank!
- 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.
- TEGAN: Eh?
- 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!
While it may not be called "index file", the use of the name appears in the story more than "information system" does. Also, Tegan's use of "index file" leads to the revelation of recursion, which is what Castrovalva (the place) is all about and is important to the resolution of the story. So it's not an error and it is important.