deleteSheet(ident uid, ident id, ident sheetId) {
  assume exists //accounts/account[uid]/assistant[id];
 
  assume exists //exercises/exercise[id]/sheets/sheet[sheetId]; # implies existence of the exercise
 
  assume size(//exercises/exercise[id]/students/student/result[sheetId]) = 0;
 
  remove //exercises/exercise[id]/sheets/sheet[sheetId];
}