⍥ — Over

Keys: Alt-1 o

f⍥g Y is f(g Y).

+/⍥, [1 2⋄3 4]   ⍝ 10

X f⍥g Y is (g X)f(g Y).

1 2 +⍥≢ 3 4 5      ⍝ 5ₓ

Compare After, which transforms only the right argument.

Inverse

With an argument fixed, X(f⍥g)⁻¹Y inverts f with g X fixed, then applies g⁻¹.

1(+⍥⊽)⁻¹8   ⍝ 3