def composition_to_s
      axes.map { |axis| CompositeUnit === axis ? "(#{axis.composition_to_s})" : axis.label.to_s }.join(" #{operator} ")
    end
def composition_to_s
      axes.map { |axis| CompositeUnit === axis ? "(#{axis.composition_to_s})" : axis.label.to_s }.join(" #{operator} ")
    end
Returns a string representation of this CompositeUnit's composition, e.g.
   ((UOM::METER * UOM::SECOND) / UOM::GRAM).composition_to_s #=> (meter * second) / gram