Bug can be entered into given branch. Bug can be entered into several branches at once. New branch of bug can be created. User can find bugs in specified branch. Attributes of bugs can differ in different branches.
Bugs can have different status and comments in different branches.
Look at the situations one and two from the case study. New branch can be created for some product from another branch. When new branch is created in CVS repository it is sometimes a good idea to branch bugs according to the CVS branch. Bug can be modified in given branch. This requirement is connected with the first requirement.