deleteExercise(ident uid, ident id) {
  assume exists //accounts/account[uid]/admin;
 
  assume exists //exercises/exercise[id];
 
  assume size(//exercises/exercise[id]/assistant) = 0;
  assume size(//exercises/exercise[id]/groups/group/tutor) = 0;
 
  assume size(//exercises/exercise[id]/students/student) = 0; # don't allow to delete if students are there
 
  remove //exercises/exercise[id];
}