Idempotence might not be the correct term but UnitFormat isn't capable to parse
back formatted unit when applied to TransformedUnit.
degree_angle [s*60/1]^-1 is such an example where UnitConverter brackets cause
parsing error. CompoundUnit seems to do soas well. Formatting on the other hand
can only handle add, multiply and rational converters.
My solution to fix the problem is to introduce toString and valueOf methods to
converters and to delegate the formatting to them allowing new unit converters
to be implemented without worrying about formatting/parsing.