Another method is to use indexing as in
/:/:0 0 0 1 0 NB. 1 is where to insert in a 4 element list
0 1 2 4 3
m=.i.5 5
b=.0 0 0 1 0 1 0
(/:/:b){m,2 5$0
0 1 2 3 4
5 6 7 8 9
10 11 12 13 14
0 0 0 0 0
15 16 17 18 19
0 0 0 0 0
20 21 22 23 24
The advantage is that you can insert anything (e.g. partial sums)
(/:/:b){m,2 5$099
0 1 2 3 4
5 6 7 8 9
10 11 12 13 14
99 99 99 99 99
15 16 17 18 19
99 99 99 99 99
20 21 22 23 24
And you can make it work columnwise easily.
/D
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