Should the extensions for a use case be an exhaustive list?

[mkeoliya]

Is it necessary for all edge cases to be included as extensions for a use case, especially for an app with a lot of permutations of commands?

For example, in AB3, should a use case for "editing a student" include an extension for

 3a. The edited student already matches an existing student. 
        3a1  AB3 shows an error message. 
        Use case ends.

Is it incorrect to exclude the above extension?

[damithc]

It's important for all edge cases to be covered (one aim of writing use cases is to discover and cater for all edge cases). However, similar edge cases can be covered using one extension rather than adding many similar extensions with minor changes.

[mkeoliya]

Thanks Prof!