**APL2 Sudoku solver**

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This is an APL (actually APL2) function to solve a Sudoku puzzle, developed
using MicroAPL APLX ver 5.1.0. "Solved" simply means a 1-9 could be
legally placed in all empty squares.
**

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The puzzle to solve is the argument to the function -- it should be a 9 x 9
numeric matrix with a 0 to indicate an empty square. The input puzzle
is not checked in any way... it is assumed to be valid.
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The function requires the index origin is set to zero. It uses a brute
force method (try all possible numbers) so it's very slow!
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The result is a nested vector. The rho of the result is the number of
solutions found (0..n) -- this should be 1 for a legal puzzle. The
content of the vector are the 9 x 9 numeric matrix solutions.
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**
r←su b;i
→⍳9∊i←0 9⊤(∊r←,⊂b)⍳0
r←b[;1⊃i],b[↑i;],,3 3↑(i-3|i)↓b
→⍳0∊⍴r←(⊂9 9)↑¨(⊂¯1-i)↑¨(⍳10)~r
r←⊃,/su¨r+⊂b
**

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