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    change _      0  = [[]]
    change []     _  = fail "can't make change"
    change (c:cs) n
       | c > n       = change cs n
       | otherwise   = map (c:) (change (c:cs) (n-c)) ++ change cs n

    > change [5,2] 16
    [[5,5,2,2,2],[2,2,2,2,2,2,2,2]]

    quickchange cs n = listToMaybe (change cs n)

...

    > quickchange [7] 16
    Nothing

    > :m +Data.Ratio
    > quickchange [1%2, 1%3] (10%6)
    Just [1%2, 1%2, 1%3, 1%3]