I've come up on several cases where a unit I was looking for was missing for a quantity. Most recently, I wanted to use "liters of ideal gas" as an amount of substance unit.

There doesn't seem to be any way to add any units to a quantity definition later on as the units are all defined through the quantity! macro. Not sure if is technically feasible as I don't really understand the implementation but something like this would be wonderful:

quantity_unit!(AmountOfSubstance, @ideal_gas_stp_liter: 0.044031629; "STPl", "STP liter", "STP liters")