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Model
effrank ←{(0≤⍺)⊃(0⌈⍺+⍴⍴⍵)(⍺⌊⍴⍴⍵)} cells ←{⊂[(-⍺ effrank ⍵)↑⍳⍴⍴⍵]⍵} assemble←{↑((1⍴¨⍨r-⍨⌈/,r←⍴∘⍴¨⍵),¨⍴¨⍵)⍴¨⍵} R←{ (m l r)←⌽3⍴⌽⍵⍵ 0=⎕NC'⍺' : assemble ⍺⍺¨ (m cells ⍵) assemble (l cells ⍺) ⍺⍺¨ (r cells ⍵) }