] 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 e. #: i.4) -: d e. #: i.4
1
   s e. #: i.4
 0 | 0
 1 | 0
 3 | 0
 4 | 0
 5 | 0
 7 | 0
10 | 0
11 | 0
12 | 0
13 | 0
14 | 0
   d e. #: i.4
0 0 1 0 0 0 1 0 1 1 0 0 0 0 0

   p=: ?.100$2e9
   q=: ?.100$1e9
   s=: q p}1 $. 2e9 ; 0 ; 2-2
   $s
2000000000
   $ s e. q
2000000000
   +/ s e. q
100