⌿
N⌿Y replicates first-axis cells.
N⌿Y
1 0⌿2 3⍴⍳6 ⍝ 1 3⍴1 2 3
f⌿Y reduces the first axis.
f⌿Y
+⌿2 3⍴⍳6 ⍝ 5 7 9
N f⌿Y performs n-wise reduction there.
N f⌿Y
2+⌿3 2⍴⍳6 ⍝ 2 2⍴4 6 8 10
Counts, identities and [K] follow /.
[K]
/