You need an addition table: +/
:
1 2 3 +/ 4 5 6
5 6 7
6 7 8
7 8 9
but by the way you describe what you want, it seems you might prefer a 0-rank addition: +"0
:
((1 2 3)&+"0) 4 5 6
5 6 7
6 7 8
7 8 9
The differences are more apparent in asymmetric additions:
1 2 +/ 3 4 5 6
4 5 6 7
5 6 7 8
((1 2)&+"0) 3 4 5 6
4 5
5 6
6 7
7 8