mapCU :: (UA a, UA b) => (a->b) -> Chunked chsz a -> Chunked chsz b
mapCU _ (CU []) = CU []
mapCU :: (UA a, UA b) => (a->b) -> Chunked chsz a -> Chunked chsz b
mapCU _ (CU []) = CU []
mapCU _ (CU []) = CU []