Sudoku←{
⍝ Solve numeric 9×9 matrix sudoku puzzle
⍝ Only works if there is 1 solution
⍝ For more competent solvers, see https://dfns.dyalog.com/n_sudoku.htm
     sc←{⍵~⍨⍳9}¨1 1↓⊂⍤⊢⌺(2 2⍴3)⊢0⍪0⍪0,0,⍵ ⍝ Possible in subcells
     r←{⊂⍵~⍨⍳9}⍤1⊢⍵                       ⍝ Possible in rows
     c←{⊂⍵~⍨⍳9}⍤1⍉⍵                       ⍝ Possible in columns
     cells←{                              ⍝ Initial contention map
         ⊃∩/(sc⌷⍨1 4 7⍸⍵),(r⌷⍨⊃⍵),c⌷⍨⊃⌽⍵
     }¨⍳9 9
     cells←⍵[⍸0≠⍵]@(⍸0≠⍵)⊢cells           ⍝ Fill with known values
     Fill←{cells←⍵
         {
             v←cells⊃⍨⊂⍵
             v~←∊c⌿⍨1=≢¨c←cells[(⊃⍵);(⍳9)~⊃⌽⍵]
             v~∊c⌿⍨1=≢¨c←cells[(⍳9)~⊃⍵;⊃⌽⍵]
         }¨⍳9 9
     }
     solved←⊃¨Fill⍣≡cells                   ⍝ Eliminate impossibilities
     PDE←{((≢⍺)⍴⍋⍋⍺⍳⍺⍪⍵)∊(≢⍵)⍴⍋⍋⍺⍳⍵⍪⍺}
     rows←∧/,(⍳9)PDE⍤1⊢solved
     columns←∧/,(⍳9)PDE⍤1⍉solved
     cells←∧/,0 1 0↓1↓{(⍳9)PDE,⍵}⌺(2 2⍴3)⊢0⍪0⍪0,0,solved
     rows∧columns∧cells:solved
     'Could not solve'
 }