MultiFocus

Optional → MultiFocus[F] — mirror of either2multifocus over the Affine miss / hit split.

Prism → MultiFocus[F] — hit becomes a pure singleton, miss becomes Alternative[F].empty.

Forget[F] ↪ MultiFocus[F] — a Fold slots into the pair carrier with X = Unit.

Iso → MultiFocus[F]. Requires Applicative[F] to broadcast the Iso's plain A focus into a singleton F[A]; the write-back picks the singleton out (throws on cardinality ≠ 1).

Same-carrier composition for MultiFocus[F] — traversal.andThen(traversal), F-parametric. Z = (Xo, F[(Xi, Int)]): the outer leftover plus, per outer focus, the inner leftover and its focus count, so composeFrom can carve the flat modified vector back into per-element slices (cursor over an array snapshot). Inners that mix in MultiFocusSingleton (the Lens bridge) take an always-hit fast path that skips the per-element inner fold. Flat focus order is the outer's traversal order, inner order within each element.

PSVec-specialised same-carrier composition. Where the generic mfAssoc body builds two intermediate List accumulators + materialises via fromList, this body writes directly into IntArrBuilder / ObjArrBuilder and stores the existential as parallel arrays in AssocSndZ, sidestepping the per-element (Xi, Int) Tuple2 the generic path pays. Fast paths for MultiFocusSingleton (always-hit, Lens bridge) and MultiFocusPSMaybeHit (Prism / Optional bridges) inners; reassembly hands each inner an O(1) PSVec.slice view. See docs/research/2026-04-29-powerseries-fold-spike.md.

ForgetfulFold[MultiFocus[F]] via Foldable[F] — folds the focus vector, discarding the leftover. Unlocks .foldMap / .headOption / .length / .exists.

ForgetfulFunctor[MultiFocus[F]] via Functor[F] — maps every focus, leftover untouched. Unlocks .modify / .replace on Traversal-family optics.

ForgetfulTraverse[MultiFocus[F], Applicative] via Traverse[F] — effectful rewrite of every focus in F's traversal order. Unlocks .modifyA / .all.

MultiFocus[F] ↪ Forget[F] — read-only escape: discard the structural leftover, keep the focused F[A]. An explicit carrier morph alongside the carrier-wide Optic.foldMap / .headOption / .length / .exists extension methods.

Structurally this is the inverse of forget2multifocus — both Composer directions ship. That's normally banned by the cats-eo Morph resolution invariant (a bidirectional pair makes Morph[Forget[F], MultiFocus[F]] ambiguous because both leftToRight and rightToLeft fire). The Composer ships anyway because:

1. The from side requires T = Unit (Forget loses the leftover, so it can't reconstruct a T ≠ Unit). Only T-Unit MultiFocus optics qualify, which the type system enforces at use sites.
2. Any chain-resolution ambiguity surfaces at forget.andThen(multifocus) / multifocus.andThen(fold) call sites — the user resolves by routing through the explicit Composer[..].to(o) form rather than .andThen.

Practical Morph fallout: if a user actually hits the ambiguity, they get a clear implicit-not- found message naming both Composers; the workaround is one extra .morph-shaped call.

MultiFocus[F] → ModifyF. Uniform Modify widening for any Functor[F].

Lens → MultiFocus[F]. Mixes in MultiFocusSingleton so the mfAssoc fast-path fires.

Generic factory: X = F[A], focus = fa, rebuild = identity.

Reinterpret a Lens whose focus is already an F[A] as a MultiFocus optic over the elements — the container the Lens reads IS the focus vector; X and the rebuild pass straight through.

Polymorphic counterpart to apply — allows focus type change (F[A] => F[B]), the Traversal.pEach analogue at the generic factory. Sound for the same reason: the rebuild is identity on the written-back F[B], so no B ever has to fit an A-shaped hole. This is what un-pins the collect flavours' element type — pApply[F, A, Row].collectWith(...) emits a different row type than it read.