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flatten! 

        # Array.flatten!

(from ruby core)
---
    array.flatten! -> self or nil
    array.flatten!(level) -> self or nil

---

Replaces each nested Array in `self` with the elements from that Array;
returns `self` if any changes, `nil` otherwise.

With non-negative Integer argument `level`, flattens recursively through
`level` levels:
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten!(1) # => [0, 1, [2, 3], 4, 5]
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten!(2) # => [0, 1, 2, 3, 4, 5]
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten!(3) # => [0, 1, 2, 3, 4, 5]
    [0, 1, 2].flatten!(1) # => nil

With no argument, a `nil` argument, or with negative argument `level`,
flattens all levels:
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten! # => [0, 1, 2, 3, 4, 5]
    [0, 1, 2].flatten! # => nil
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten!(-1) # => [0, 1, 2, 3, 4, 5]
    a = [ 0, [ 1, [2, 3], 4 ], 5 ]
    a.flatten!(-2) # => [0, 1, 2, 3, 4, 5]
    [0, 1, 2].flatten!(-1) # => nil

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