Function reduceCollection

src/compute-engine/library/collections.ts:3080–3093

(
  collection: Expression,
  fn: (acc: T, next: Expression) => T | null,
  initial: T
)

// `Map((p1, …, pk) ↦ Tuple(…), s1, …, sk)` as built by `lazyBroadcastMap` for
// a large `PointList` — returning the source collection the projected slot
// binds to. `PointX(PointList(a, b, c))` is then just `a`: no per-element
// Tuple construction and `At` extraction on drain (Tycho item 52). Sound only
// when the slot is a plain parameter reference, every source is an INDEXED
// collection (the lazy-transpose contract — a `Map` over a `Set`, though it
// can look identical, must keep the generic path's indexed-`List` result),
// and every source has the same known count (`Map` zips to the shortest
// source, so projecting one source of a ragged zip would yield extra
// elements). Anything else returns `undefined` and the caller falls through
// to the generic path.
//
// Under `numericApproximation` the projection is returned as its lazy `.N()`
// form: the source itself is the EXACT collection (the transpose body's
// `N(…)` wrap belongs to the whole tuple, and is discarded with it), so
// returning it bare would let `PointX(pts).N()` yield exact elements —
// violating `x.N() ≡ x.evaluate().N()` parity.
function projectLazyPointList(
  xs: Expression,