 Is this mixed amount exactly zero, ignoring its display precision?
-- i.e. does it have zero quantity with no price, zero quantity with a total price (which is also zero),
-- and zero quantity for each unit price?
mixedAmountIsZero :: MixedAmount -> Bool
mixedAmountIsZero (Mixed ma) = all amountIsZero ma

--
mixedAmountIsZero :: MixedAmount -> Bool
mixedAmountIsZero (Mixed ma) = all amountIsZero ma
mixedAmountIsZero (Mixed ma) = all amountIsZero ma