-- `flt fn (window n xs)` and `map fn (window n xs)` are *fused* at the
-- bytecode emitter: the VM walks xs once with stride 1, reusing a single
-- scratch list as the window for each call into the predicate / mapper.
-- Without this fusion, an idiomatic `flt all-hydro (window 15 (chars s))`
-- over an 11.4M-residue string would allocate ~170M small lists and run ~6x
-- slower than the hand-rolled imperative form (bioinformatics rerun5).
-- Tree, VM, and Cranelift all return the same output; the in-place reuse
-- fast path lives in the VM dispatcher's `OP_WINDOW_VIEW` arm (and the
-- corresponding Cranelift helper).
hassum w:L n>b;>(sum w) 5
-- flt keeps the windows whose elements sum > 5. Only [3,4] qualifies.
keep>L (L n);flt hassum (window 2 [1, 2, 3, 4])
-- map applies the mapper to each window. Stride-1 windows of [1..5] are
-- [1,2,3], [2,3,4], [3,4,5]; their sums are 6, 9, 12.
sums>L n;map sum (window 3 [1, 2, 3, 4, 5])
-- An empty source list, an n larger than the list, and an unrelated `window`
-- call (whose result escapes, so fusion can't fire) must all still produce
-- the right answer.
empty>L (L n);flt hassum (window 3 [])
toobig>L n;map sum (window 10 [1, 2, 3])
escape>L (L n);window 3 [10, 20, 30, 40]
-- run: keep
-- out: [[3, 4]]
-- run: sums
-- out: [6, 9, 12]
-- run: empty
-- out: []
-- run: toobig
-- out: []
-- run: escape
-- out: [[10, 20, 30], [20, 30, 40]]