This class represents an answer or a response to a question on the CAGE Assessment Instrument. This abstract class exists primarily to re-define the generic AssessmentEvent.responseItem.answer property to the CAGE.responseItem.answer. In other words, of all the possible answers on all the possible kinds of assessment instruments, we are explicitly identifying the possible answers to the four CAGE questions. Note that because this class is abstract, implementers would likely not instantiate this class, but rather the four concrete sub classes.