def to_xml_string(str = '')
      return if empty?
      str << '<definedNames>'
      each { |defined_name| defined_name.to_xml_string(str) }
      str << '</definedNames>'
    end
def to_xml_string(str = '')
      return if empty?
      str << '<definedNames>'
      each { |defined_name| defined_name.to_xml_string(str) }
      str << '</definedNames>'
    end
creates the DefinedNames object
 Serialize to xml
 @param [String] str
 @return [String]