unregisterStudent(ident uid, ident id, ident studentId) {
  assume exists //exercises/exercise[id] && //exercises/exercise[id]/open && 
         exists //accounts/account[uid]/student && //accounts/account[uid]/student/id = studentId
      || exists //accounts/account[uid]/assistant[id];
 
  assume exists //exercises/exercise[id]/student[studentId]; # implies the exercise exists
 
  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];
}