-- Negative indices on slice-shaped builtins count from the end of the
-- list or text, matching Python semantics already in place for `at xs i`.
--
-- slc xs s e accepts negative s and e. `slc xs -1 (len xs)` is the
-- last element as a 1-element list; `slc xs 0 -1` drops
-- the last element; bounds clamp, never wrap.
-- take n xs with n<0 keeps all but the last |n|. Equivalent to
-- Python's xs[:n]. `take -1 [10,20,30]` is [10,20].
-- drop n xs with n<0 keeps only the last |n|. Equivalent to
-- Python's xs[n:]. `drop -1 [10,20,30]` is [30].
--
-- Closes the quant-trader fencepost (`@i 1..np` loops produce np-1 results,
-- so the equity curve ends at length np; `at eq np` errored, and the
-- workaround was `npm=- np 1`). With negative indices everywhere, the same
-- pattern is `slc eq 0 -1` — no separate binding, no off-by-one risk.
last-element xs:L n>L n;slc xs -1 (len xs)
drop-last xs:L n>L n;slc xs 0 -1
keep-last-two xs:L n>L n;slc xs -2 (len xs)
all-but-last xs:L n>L n;take -1 xs
only-last-two xs:L n>L n;drop -2 xs
trim-edges s:t>t;slc s 1 -1
-- run: last-element [10,20,30,40,50]
-- out: [50]
-- run: drop-last [10,20,30,40,50]
-- out: [10, 20, 30, 40]
-- run: keep-last-two [10,20,30,40,50]
-- out: [40, 50]
-- run: all-but-last [10,20,30,40,50]
-- out: [10, 20, 30, 40]
-- run: only-last-two [10,20,30,40,50]
-- out: [40, 50]
-- run: trim-edges "(quoted)"
-- out: quoted