changeResult(string uid, string id, string studentId, string sheetId, double points) {
  assume exists //accounts/account[uid]/assistant[id] || 
         exists //accounts/account[uid]/tutor[id]/group[//exercises/exercise[id]/students/student[studentId]/group];
 
  assume exists //exercises/exercise[id]/students/student[studentId]; # implies the existence of the exercise
  assume exists //exercises/exercise[id]/sheets/sheet[sheetId]; # dito
 
  assume points >= 0 && points <= //exercises/exercise[id]/sheets/sheet[sheetId]/maxPoints;
 
  assume exists //exercises/exercise[id]/students/student[studentId]/result[sheetId];
 
  update //exercises/exercise[id]/students/student[studentId]/result[sheetId]/points points;
}