challenge-055/richard-park/apl/Pmat.aplf


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 Pmat←{⎕ML←1             ⍝ dfns pmat: Permutation matrix of ⍳⍵.
     {                   ⍝ perms of ⍳⍵:
         1≥⍴⍵:↑,↓⍵       ⍝ short vector: done.
         ↑⍪/⍵,∘∇¨⍵∘~¨⍵   ⍝ items prefix sub-perms of remainder.
     }⍳⍵                 ⍝ permutations of identity perm.
 }