-- `lst xs i v` is "list set at index" (3 args, returns updated list).
-- It is NOT "last element". Agents often misread the name and reach for
-- `lst xs`, hitting an arity error. The canonical "last element" is
-- `at xs -1`, which mirrors Python-style negative indexing.
-- Wrong: `lst xs` would error with ILO-T006 + suggestion pointing here.
-- Right (last element):
last xs:L n>n;at xs -1
-- Right (set index i to v): replace index 1 with 99.
setone xs:L n>L n;lst xs 1 99
-- run: last [10,20,30]
-- out: 30
-- run: setone [10,20,30]
-- out: [10, 99, 30]