Monadic Back Slash Bar is not defined
mat
1 2 3 4
5 6 7 8
9 10 11 12
1 0 2 1 ⍀ mat
1 2 3 4
0 0 0 0
5 6 7 8
5 6 7 8
9 10 11 12
Back Slash Bar is a Monadic operator with a Dyadic operand
+⍀ mat
1 2 3 4
6 8 10 12
15 18 21 24