Computes all possible combinations of `n`

objects taken `k`

at a
time.

`Combinations(n, k)`

- n
a number.

- k
a number less than or equal to

`n`

.

Returns a matrix containing the possible combinations of `n`

objects taken `k`

at a time.

```
Combinations(5,2)
#> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
#> 1 1 2 1 2 3 1 2 3 4
#> N 2 3 3 4 4 4 5 5 5 5
# The columns in the matrix list the values of the 10 possible
# combinations of 5 things taken 2 at a time.
```