The magic function for the general case of key, f⌸

{
  0=⎕nc'⍺':⍵ ∇ ⍳≢⍵    ⍝ f⌸ x ←→ x f⌸ ⍳≢x
  ⍺ ⍺⍺{⍺ ⍺⍺ ⍵}{
    ⎕ml←1             ⍝ needed by ↑⍵ and ⍺⊂⍵
    j←⍋i⊣⎕dr i←⍳⍨⍺
    ↑ (⊂[1↓⍳⍴⍴⍺]((⍳≢i)=⍳⍨i)⌿⍺) ⍺⍺¨
                         (2≠/¯1,i[j]) ⊂[⎕io] ⍵⌷⍨⊂j
  }⍵
}