M   =:  4 4$0 1 5 6 2 4 7 12 3 8 11 13 9 10 14 15   NB.  Bill's 
example
           zz  =:  ($ $ , i.~ [: ; <@:|.`</.)@:i.@:,~
           M -: zz 4
        1
           
           

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