Great, thanks. Also are we allowed to create a relationship between subclasses? I have set up the Client superclass with patient and customer subclasses, so am I able to create a customerID (seperate from the ClientID), that I can link to the patient table so that the patient can have a record who is paying for their appointment? And just to clarify, I know we aren't meant to note foreign keys, but do we include them as regular attributes in the entities? For example can I include the patients identifier in appointments entity? Thank you