luhn.apple

-- 16-digit for credit card
λxs.
  { digitSum ← [?x>10,.x-9,.x]
  ; t ← (+)/ [digitSum (x*y)]`(~(}:xs)) (}: (cyc. ⟨2,1::int⟩ 8))
  ; 10-(t|10)=}.xs
  }