registerStudent(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];
  assume not exists //exercises/exercise[id]/students/student[studentId];
 
  insert //exercises/exercise[id]/students <student id=[studentId] />;
}