-- enumerate xs: pair each element with its index, returning [[i, v], ...].
-- "map with index" appears in every program — destructure the pair to use it.
idx xs:L n>L (L _);enumerate xs
words xs:L t>L (L _);enumerate xs
-- run: idx [10,20,30]
-- out: [[0, 10], [1, 20], [2, 30]]
-- run: words ["a","b","c"]
-- out: [[0, a], [1, b], [2, c]]