implement `combinations` method about iterum

[xgbuils]

The idea that I have to implement the combinations is iterating over combination values evaluating the minimum possible information. For example, if I want to evaluate the combinations of (1, 2, 3, 4, 5, 6) over 3. The first combination is:

(1 2 3)

Then, I need the next value 4 to create new combinations:

(1 2 4)
(1 3 4)
(2 3 4)

When it is not possible to create more combinations, I consume the next value 5

(1 2 5)
(1 3 5)
(2 3 5)
(1 4 5)
(2 4 5)
(3 4 5)

When it is not possible to create more combinations, I consume the next value 6

(1 2 6)
(1 3 6)
(2 3 6)
(1 4 6)
(2 4 6)
(3 4 6)
(1 5 6)
(2 5 6)
(3 5 6)
(4 5 6)

I think I found a pattern to implement this iteration.