Ex. 3 (15 pts) Suppose we have two kinds of doctors: hospital doctors and family physicians. In addition to the doctor's id number, name, specialty, and years of experience, we want to record the hospital name for the hospital doctors, and the office address for the family physicians. There can be doctors that are working in a hospital who are at the same time family physicians in their free time. Also there can be doctors for whom we don't know whether they are working in a hospital and/or whether they are family physicians or whether they are not working at all