The combination function yields the number of ways n distinct items can be
chosen for k spots, when the order in which they are chosen does
not matter--that is, choosing ABCDE is equivalent to choosing BAEDC. In other words, we use
the combination function when all spots are equivalent.
Example: If Jim has 12 shirts, and needs to pack 7 for vacation, how
many different combinations of shirts can he pack?
C(12, 7) =
= 792 different combinations of shirts.