↣ — Before

Keys: Alt-.

f↣g Y is (f Y)g Y, where f prepares the left argument of g. Operators bind from the left. For example, 3↣<↣# is (3↣<)↣#, which keeps the items greater than 3.

-↣+ 5              ⍝ 0
3↣<↣#2 7 1 8       ⍝ 7 8
' '↣≠↣⊆"a bc  d"   ⍝ "a" "bc" "d"

X f↣g Y is (f X)g Y.

10 -↣+ 3           ⍝ ¯7

A↣f binds the left argument. The bound function takes one argument. A train binds the same way: 10- y is (10↣-)y.

10↣- 3             ⍝ 7
32+1.8↣× 100       ⍝ 212

See After, Atop / Rank and Over.

Inverse

With an array operand, (X↣f)⁻¹Y is X f⁻¹ Y. With two functions, X(f↣g)⁻¹Y inverts g with f X fixed on the left.

(2↣×)⁻¹6    ⍝ 3
3(-↣+)⁻¹7   ⍝ 10