def format
      raise FormatError.new(self) unless formattable?

      REXML::Element.new("extension").tap do |ext|
        ext.attributes["name"] = @name
        ext.attributes["prefix"] = @prefix
        ext.attributes["uri"] = @uri
      end
    end
def format
      raise FormatError.new(self) unless formattable?

      REXML::Element.new("extension").tap do |ext|
        ext.attributes["name"] = @name
        ext.attributes["prefix"] = @prefix
        ext.attributes["uri"] = @uri
      end
    end
Format as a XML element.

 @return [REXML::Element]
   XML element