code: def set_default(self, value):
          
          if value is None:
              self.__default = value
          elif not isinstance(value, str):
              raise TypeError("Default must be set to a String")
          else:
              self.__default = value
docs: Setter for 'default' field.
          :param value - a new value of 'default' field. Must be either None (default is optional according to
          BPMN 2.0 XML Schema) or String.