def +(x)
      x = Unit.cast(x)
      check_operable2(x)
      assert_same_dimension(x)
      Unit.new(@factor+x.factor,@dim.dup)
    end
def +(x)
      x = Unit.cast(x)
      check_operable2(x)
      assert_same_dimension(x)
      Unit.new(@factor+x.factor,@dim.dup)
    end
Addition of units.
 Both units must be operable and conversion-allowed.
 @param  [Phys::Unit, Numeric] x  other unit
 @return [Phys::Unit]
 @raise  [Phys::UnitError] if unit conversion is failed.