HEADERS_END

A Combination is an unordered subset of a set.

For example, the 10 possible combinations of 3 letters from the set of

AEI, AEO, AEU, AIO, AIU, AOU, EIO, EIU, EOU and IOU.

is incorrect as the code is

Order does not matter, so the choices "AEI" and "EAI" (for example) are considered to be the same.

The calculation of the number of combinations makes use of the factorial notation,

binomial coefficients and Pascal's triangle.