-- flatmap: map then flatten one level. Use when a function returns a list
-- per element and you want a single flat list of results.
-- Signature: flatmap fn:F a (L b) xs:L a > L b
-- Repeat each number n times: 2 -> [2, 2], 3 -> [3, 3, 3]
rep n:n>L n;xs=[];@i 0..n{xs=+=xs n};xs
-- Apply rep to every element and flatten one level.
expand xs:L n>L n;flatmap rep xs
-- run: expand [1,2,3]
-- out: [1, 2, 2, 3, 3, 3]