-- Call-form expressions are allowed on both sides of `..` in @ loops.
-- Saves the intermediate-binding tax that personas kept tripping over:
-- `n=len xs;@j 0..n` collapses to `@j 0..len xs`.
-- End bound is a 1-arg call: sum the indices of xs.
sum-indices xs:L n>n;s=0;@j 0..len xs{s=+s j};+s 0
-- End bound is a 2-arg call: `at ys 0` is the first element of ys.
sum-to-first ys:L n>n;s=0;@j 0..at ys 0{s=+s j};+s 0
-- Start bound is a call: iterate from xs[0] up to len xs.
range-from-first xs:L n>L n;out=[];@j at xs 0..len xs{out=+=out j};out
-- Mixed: prefix-binop start with call-form end.
slide i:n xs:L n>L n;out=[];@j +i 2..len xs{out=+=out j};out
-- run: sum-indices [1,2,3]
-- out: 3
-- run: sum-to-first [4,9,9,9]
-- out: 6
-- run: range-from-first [2,9,9,9]
-- out: [2, 3]
-- run: slide 3 ["a","b","c","d","e","f"]
-- out: [5]