Calculating Number of Distinct x Tuples from a Set A

[AKG]

BicycleTree, you are not allowed to swap a key that has already been swapped.

This is what is meant when he said:

Think of it as a physical system containing 10 physical elements, if 1 is used in one pair, it can't be reused in another pair or again in the same pair.
so, (2, 2) can't be used and (2, 3), (2, 4) can't be used.

 

[BicycleTree]

Yeah, but that doesn't strictly apply to the keyboard example. It's not really important.