def inverse
      check_operable
      dims = dimension_uop{|a| -a}
      Unit.new(Rational(1,self.factor), dims)
    end
def inverse
      check_operable
      dims = dimension_uop{|a| -a}
      Unit.new(Rational(1,self.factor), dims)
    end
Inverse of units.
 This unit must be operable.
 @param  [Phys::Unit, Numeric] unit
 @return [Phys::Unit]
 @raise  [Phys::UnitError] if not operable.