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