# `⇄` — Inverse pair

Keys: `Alt-4`

`f⇄g` calls `f`. `(f⇄g)⁻¹` is `g⇄f`.

``` bpl
f←{≥⍵}⇄{≤⍵} ⋄ f5            ⍝ 6
f←{≥⍵}⇄{≤⍵} ⋄ f⁻² 5        ⍝ 3
```

Dyadic inversion holds `X` fixed: `X g (X f Y)` must recover `Y`.

``` bpl
f←{⍺+⍵}⇄{⍵-⍺} ⋄ 3 f⁻¹ 8       ⍝ 5
f←{⍺+⍵}⇄{⍵-⍺} ⋄ (3↣f)⁻¹8      ⍝ 5
```

Left binding preserves the pair. For right binding, pair the bound
monad: `(f↢A)⇄undo`.

See [Power](power.qmd) and [Under](under.qmd).