def to_s(attributes=nil)
      content = printable_content(attributes)
      content_s = content.pp_s(:single_line) unless content.empty?
      "#{print_class_and_id}#{content_s}"
    end
def to_s(attributes=nil)
      content = printable_content(attributes)
      content_s = content.pp_s(:single_line) unless content.empty?
      "#{print_class_and_id}#{content_s}"
    end
Prints this domain object in the format:
   class_name@object_id{attribute => value ...}
 The default attributes include identifying attributes.

 @param [<Symbol>] attributes the attributes to print
 @return [String] the formatted content