How to splice array and merge back in Ruby? : Say I have 2D array like so:

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    [ 3,    4,   8,   12 ]
    [ 2,    6,   7,   16 ]
    [ 1,   10,  11,   15 ]
    [ 5,    9,  13,   14 ]

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I want to `shift` 6 in **`[1][1]`** and `unshift` it into **`[2][2]`**

So that I can get the following:

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    [ 3,    4,   8,    12 ]
    [ 2,    7,   11,   16 ]
    [ 1,    6,   10,   15 ]
    [ 5,    9,   13,   14 ]

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I thought I'd `splice(1,1)` 2nd and 3rd array to get:
    
    a: [6, 7]
    b: [10, 11]

and then do: 
     
    b.unshift(a.shift)
    a << b.pop