Monadic Iota Underbar means
⍸ 1 0 0 1 1
1 4 5
bmat
0 1 0
1 0 1
⍸ bmat
┌───┬───┬───┐
│1 2│2 1│2 3│
└───┴───┴───┘
Dyadic Iota Underbar means
'AEIOU' ⍸ 'DYALOG'
1 5 1 3 4 2
2 4 6 ⍸ 1 2 3 4 5 6 7
0 1 1 2 2 3 3
mat
1 2
3 4
5 6
mat ⍸ 3 3
1
mat ⍸ 3 5
2