old_contents: from som.vmobjects.abstract_object import AbstractObject
  
  
  class ObjectWithoutFields(AbstractObject):
  
      _immutable_fields_ = ["_class"]
  
      def __init__(self, obj_class):
          self._class = obj_class
  
      def get_class(self, universe):
          return self._class
  
      def set_class(self, value):
          self._class = value
  
      def get_number_of_fields(self):
          return 0
  
new_contents: from som.vmobjects.abstract_object import AbstractObject
  
  
  class ObjectWithoutFields(AbstractObject):
  
      _immutable_fields_ = ["_class"]
  
      def __init__(self, obj_class):
          assert obj_class is None or isinstance(obj_class, ObjectWithoutFields)
          self._class = obj_class
  
      def get_class(self, universe):
          return self._class
  
      def set_class(self, value):
          self._class = value
  
      def get_number_of_fields(self):
          return 0
  
subject: Add assertion for obj_class