-- ct fn xs -> n: count elements where predicate returns true.
--
-- Token-equivalent to `len (flt fn xs)` but skips the intermediate L b
-- filter-result allocation. Motivating workload: bioinformatics rerun6
-- ran `tm=ct has-tm seqs` over 20k proteins, replacing
-- `tm=len (flt has-tm seqs)` and saving an L b alloc the size of the
-- whole keepers list.
--
-- Named `ct` (not `cnt`) because `cnt` is reserved as the loop continue
-- keyword. Two chars beats the three-char ask in the persona log on
-- token economy.
pos x:n>b;>x 0
-- Count positives in a mixed list.
pcount>n;ct pos [-3,0,2,4,-1,5]
-- Closure-bind variant: count elements above a threshold.
gt x:n c:n>b;>x c
abovec>n;ct gt 4 [1,5,3,8,2,7]
-- Empty list returns 0; no allocation, no error.
empty>n;ct pos []
-- run: pcount
-- out: 3
-- run: abovec
-- out: 3
-- run: empty
-- out: 0