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Story Description
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Once the Grader looks at the details of a submission, possibly consisting of
several files, she/he can select one file and can begin to mark this
particular files (adding comments, etc). The submitted file from the
student-group/submission-group should not be modifiable.

Note: Maybe add link to move quickly to next submission assigned to the
Grader.

Tests
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* Switch between different files of a submission and test if changes or
uncommitted modification in comments are preserved, or a warning pops-up.