apl: commute f⍨ (+10 tests, 173/173)

plans/apl-on-sx.md

@@ -78,7 +78,7 @@ Core mapping:
 - [x] Each `f¨` — applies `f` to each scalar/element
 - [x] Outer product `∘.f` — `1 2 3 ∘.× 1 2 3` ↦ multiplication table
 - [x] Inner product `f.g` — `+.×` is matrix multiply
-- [ ] Commute `f⍨` — `f⍨ x` ↔ `x f x`, `x f⍨ y` ↔ `y f x`
+- [x] Commute `f⍨` — `f⍨ x` ↔ `x f x`, `x f⍨ y` ↔ `y f x`
 - [ ] Compose `f∘g` — applies `g` first then `f`
 - [ ] Power `f⍣n` — apply f n times; `f⍣≡` until fixed point
 - [ ] Rank `f⍤k` — apply f at sub-rank k
@@ -118,6 +118,7 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-06: Phase 4 step 6 — commute f⍨ (apl-commute monadic dup, apl-commute-dyadic swap); 173/173 tests
 - 2026-05-06: Phase 4 step 5 — inner product f.g (apl-inner); +.× matrix multiply, ∧.= equal-vectors; 163/163 tests
 - 2026-05-06: Phase 4 step 4 — outer product ∘.f (apl-outer); rank-doubling result shape = a-shape++b-shape; 151/151 tests
 - 2026-05-06: Phase 4 step 3 — each f¨ (monadic apl-each + dyadic apl-each-dyadic); scalar broadcast both sides; 139/139 tests