⍥
f⍥g Y is f(g Y).
f⍥g Y
f(g Y)
(+/⍥,)2 2⍴⍳4 ⍝ 10
X f⍥g Y is (g X)f(g Y).
X f⍥g Y
(g X)f(g Y)
1 2 +⍥≢ 3 4 5 ⍝ 5x
Compare Compose, which transforms only the right argument.