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Story Description
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In order to make marking of submissions easier, the Grader can switch from one
submission to another by clicking on a "next submission" link (or a link with
a similar title). The Grader should be warned about any unsaved modifications.

Tests
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* Test if MarkUs warns the Grader about unsaved modifications 
* Show that it MarkUs changes only to submission assigned to the according
Grader.