Monadic Slash Bar is not defined
Replicate First (Compress First)
mat
1 2 3 4
5 6 7 8
9 10 11 12
1 0 2 ⌿ mat
1 2 3 4
9 10 11 12
9 10 11 12
Slash Bar is a Monadic operator with a Dyadic operand
Reduce First, Reduce First N-Wise
+⌿ mat
15 18 21 24
2 +⌿ mat ⍝ pair-wise
6 8 10 12
14 16 18 20