] d=: ?. 15 2$4 
0 3
1 2
0 0
2 2
3 1
2 3
0 0
2 2
0 1
0 1
2 2
3 3
2 0
2 1
2 3
   s=: $. d

   (~. s) -: ~. d
1
   ~.s
0 1 | 3
1 0 | 1
1 1 | 2
3 0 | 2
3 1 | 2
4 0 | 3
4 1 | 1
5 0 | 2
5 1 | 3
6 1 | 1
7 0 | 3
7 1 | 3
8 0 | 2
9 0 | 2
9 1 | 1
   ~. d
0 3
1 2
0 0
2 2
3 1
2 3
0 1
3 3
2 0
2 1

   p=: ?.100$1e9
   q=: (?.100$20){?.20 5$10
   s=: q p}1 $. 1e9 5 ; 0 ; 2-2
   $s
1000000000 5
   t=: ~. s
   $t
21 5
   ($t)   -: $ ~.0,q
1
   (2$.t) -: ,0
1
   (3$.t) -: 0
1
   (4$.t) -: ,. (i.#t)-.{.(i.1+#p) -. p
1
   (5$.t) -: ~.q/:p
1