 Add an Amount to a MixedAmount, normalising the result.
maAddAmount :: MixedAmount -> Amount -> MixedAmount
maAddAmount (Mixed ma) a = Mixed $ M.insertWith sumSimilarAmountsUsingFirstPrice (amountKey a) a ma

--
maAddAmount :: MixedAmount -> Amount -> MixedAmount
maAddAmount (Mixed ma) a = Mixed $ M.insertWith sumSimilarAmountsUsingFirstPrice (amountKey a) a ma
maAddAmount (Mixed ma) a = Mixed $ M.insertWith sumSimilarAmountsUsingFirstPrice (amountKey a) a ma