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repeated_combination 

        # Array.repeated_combination

(from ruby core)
---
    array.repeated_combination(n) {|combination| ... } -> self
    array.repeated_combination(n) -> new_enumerator

---

Calls the block with each repeated combination of length `n` of the
elements of `self`; each combination is an Array; returns `self`. The
order of the combinations is indeterminate.

When a block and a positive Integer argument `n` are given, calls the
block with each `n`-tuple repeated combination of the elements of
`self`. The number of combinations is `(n+1)(n+2)/2`.

`n` = 1:
    a = [0, 1, 2]
    a.repeated_combination(1) {|combination| p combination }

Output:
    [0]
    [1]
    [2]

`n` = 2:
    a.repeated_combination(2) {|combination| p combination }

Output:
    [0, 0]
    [0, 1]
    [0, 2]
    [1, 1]
    [1, 2]
    [2, 2]

If `n` is zero, calls the block once with an empty Array.

If `n` is negative, does not call the block:
    a.repeated_combination(-1) {|combination| fail 'Cannot happen' }

Returns a new Enumerator if no block given:
    a = [0, 1, 2]
    a.repeated_combination(2) # => #<Enumerator: [0, 1, 2]:combination(2)>

Using Enumerators, it's convenient to show the combinations and counts
for some values of `n`:
    e = a.repeated_combination(0)
    e.size # => 1
    e.to_a # => [[]]
    e = a.repeated_combination(1)
    e.size # => 3
    e.to_a # => [[0], [1], [2]]
    e = a.repeated_combination(2)
    e.size # => 6
    e.to_a # => [[0, 0], [0, 1], [0, 2], [1, 1], [1, 2], [2, 2]]

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