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Story Description
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When all submissions are in marking-status "marked" the instructor can release
marks, so that students can view marks for their submissions. Also, he/she can
unrelease marks. Once marks have been released, they cannot be changed. In
order to change marks upon release the instructor has to unrelease marks
first. 

Tests
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* Test if Graders can change marks when submissions for an assignment are
released.

Questions
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* Should the instructor be able to release marks before all submissions are in
"marked" status?