the top comment trick [ n, 1, n + 1, 0 ][n % 4] can be implemented in J as following :

   f =: ]`1:`>:`0:@.(4&|)"0

   (,. ; #: ; [: #: f) i.16

 0    0 0 0 0    0 0 0 0    
 1    0 0 0 1    0 0 0 1    
 2    0 0 1 0    0 0 1 1    
 3    0 0 1 1    0 0 0 0    
 4    0 1 0 0    0 1 0 0    
 5    0 1 0 1    0 0 0 1    
 6    0 1 1 0    0 1 1 1    
 7    0 1 1 1    0 0 0 0    
 8    1 0 0 0    1 0 0 0    
 9    1 0 0 1    0 0 0 1    

Hello fellow J programmer. In statistic you can estimate the population size by coloring fishes that you put in a lake and some time later you fish in that lake, the proportion of colored fishes allow you to estimate the size of the population. Four month ago I posted some J identities [1] and you were the only one which commented, that means that in this capture I only got one fish and it was the colored fish. This imply that there must be very few J programmers HN, or more precisely, very few of them that post J related material.

The parent's comment (also mine) has a style that was designed not to scare non J programmers. One should also consider that some people dislike J code so downvotes are the usual result except when the post provides some additional insight.

Finally, thank you for this small J lesson, is a pleasure to find here fellow J programmers.

[1] https://news.ycombinator.com/item?id=42859077