def to_s format=:symbol
      if format == :name
        unit_string = @value == 1 || @value == -1 ? @unit.name : @unit.pluralized_name 
      else
        unit_string = @unit.send format 
      end

      string =  "#{@value}"
      string += " #{unit_string}" unless unit_string.empty?

      string
    end
def to_s format=:symbol
      if format == :name
        unit_string = @value == 1 || @value == -1 ? @unit.name : @unit.pluralized_name 
      else
        unit_string = @unit.send format 
      end

      string =  "#{@value}"
      string += " #{unit_string}" unless unit_string.empty?

      string
    end
Returns a string representation of the quantity, using the unit symbol