Function count_combinations 

pub fn count_combinations(
    n_total: u64,
    n_choose: u64,
) -> Result<u64, CombinatoricsError>

Count combinations - the number of ways to pick n_choose items from n_total, where order does not matter.

Formula

C(n, r) = n! / (r! * (n-r)!)

Not defined for r > n.