How to show "OR" statement in sequence diagram?

Question

When drawing a sequence diagram , I saw that I need to implement an "OR" statement.Such as,

CUSTOMER can pay the bill to either WAITER or CASHIER.

Now,I have a paidBill() function which can pay either waiter or cashier. Please help to draw this.

Answer 1

This is not really "OR", because that would imply that you can pay both Waiter and Cashier at the same time ("OR" is one, the other, or both). So what you want instead is if/else. This is denoted by combined fragments.