code: def register(cls):
      
      if not issubclass(cls, Entity):
          raise ValueError("Class must be a subclass of abilian.core.entities.Entity")
  
      SupportAttachment.register(cls)
      return cls
docs: Register an :class:`~.Entity` as a attachmentable class.
  
      Can be used as a class decorator:
  
      .. code-block:: python
  
        @attachment.register
        class MyContent(Entity):
            ....