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unregisterStudent(ident uid, ident id, ident studentId) { assume //accounts/account[uid]/student/id = studentId; assume exists //exercises/exercise[id]/students/student[studentId]; # implies the exercise exists assume //exercises/exercise[id]/open; assume size(//exercises/exercise[id]/students/student[studentId]/result) = 0; # don't allow unregistering if results are there remove //exercises/exercise[id]/students/student[studentId]; }