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 ⍵)
}