unregisterStudent(ident uid, ident id, ident studentId) {
  assume exists //accounts/account[uid]/student;
  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];
}