Draw the entity-relationship diagram for this patient appointment system. The business rules of this system are the following:
A doctor can be scheduled for many appointments, but may not have any appointments scheduled at all. Each appointment is scheduled with exactly one doctor. A patient can schedule one or more appointments. One appointment is scheduled with exactly one patient. An appointment must generate exactly one bill, and a bill is generated by only one appointment. One payment is applied to exactly one bill, and one bill can be paid off over time by several payments. A bill can be outstanding, meaning that it has not been paid at all. One patient can make many payments, but a single payment is made by only one patient. Some patients are insured by an insurance company. If they are insured, they can carry insurance with only one insurance company. An insurance company can have many patients carry their policies. For patients who carry insurance, the insurance company will make payments, with each single insurance payment made by exactly one insurance company.