 Sum two MixedAmount.
maPlus :: MixedAmount -> MixedAmount -> MixedAmount
maPlus (Mixed as) (Mixed bs) = Mixed $ M.unionWith sumSimilarAmountsUsingFirstPrice as bs

--
maPlus :: MixedAmount -> MixedAmount -> MixedAmount
maPlus (Mixed as) (Mixed bs) = Mixed $ M.unionWith sumSimilarAmountsUsingFirstPrice as bs
maPlus (Mixed as) (Mixed bs) = Mixed $ M.unionWith sumSimilarAmountsUsingFirstPrice as bs