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