packN←{                     ⍝ Null packing.

    cmp←{
        mask←,⍵≠1↑0⍴⍵       ⍝ mask of non-nulls.
        (⍴⍵)mask(mask/,⍵)   ⍝ shape mask non-nulls.
    }

    exp←{
        shape mask items←⍵
        shape⍴mask\items
    }

    ⍺←1 ⋄ ⍺:cmp ⍵ ⋄ exp ⍵   ⍝ compress or expand.
}

code_colours

test script

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