def div(other)
      if Quantity===other
        @value.div( @unit.convert(other) )
      else
        self.class.new( @value.div(other), @expr, @unit )
      end
    end
def div(other)
      if Quantity===other
        @value.div( @unit.convert(other) )
      else
        self.class.new( @value.div(other), @expr, @unit )
      end
    end
Division without Modulo.
 * If the +other+ param is Phys::Quantity,
   +other+ is converted to the unit of +self+,
   and returns +div+ of values.
 * If the +other+ param is *not* Phys::Quantity,
   +other+ is regarded as dimensionless,
   and returns +div+ of Phys::Quantity.
 @param  [Object] other
 @return [Object] div
 @raise  [Phys::UnitError] if unit conversion is failed.