how to calculate the size of the set in Footnote 1 ？

[moses1994]

Your blog is really good. Thank you for your interpretation.
I'd like to ask about how to calculate the size of the set in Footnote 1.
As noted in footnote 1, 'for a Y of length U without any repeat characters and an X of length T the size of the set is {T + U T - U}', I don't understand why the size of the set is a combination of (T - U) out of (T + U).
c((T - U), (T + U))=p((T - U), (T + U))/(T - U)!
I'm looking forward to your reply. Thank you very much.

[awni]

Take a look at #36.