This class represents an answer or a response to a question on the Patient Health Questionnaire (PHQ-9) Assessment Instrument. This abstract class exists primarily to re-define the generic AssessmentEvent.responseItem.answer property to the PHQ9Assessment.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 nine PHQ-9 questions. Note that because this class is abstract, implementers would likely not instantiate this class, but rather the nine concrete sub classes.