Even I have the same dilemma that Jonathan has. For instance, suppose your are designing an online shopping system. In given requirements, you are provided following use-cases:
Add/Remove Items to/from shopping cartCheckout
But when you start analyzing, you realize that
you should also provide a use-case to allow saving a shopping cart that can be checked-out later.
What should one do in such cases? Just stick with the given use-cases or design for use cases that are not mentioned in the assignment?