addResult(ident uid, ident id, ident studentId, ident sheetId, double points) {
  assume exists /account[uid]/assistant[id] 
      || exists /account[uid]/tutor[id]/group[/exercise[id]/student[studentId]/group];
 
  assume exists /exercise[id]/student[studentId]; # implies the existence of the exercise
  assume exists /exercise[id]/sheet[sheetId]; # dito
 
  assume not exists /exercise[id]/student[studentId]/result[sheetId];
 
  assume points >= 0 && points <= /exercise[id]/sheet[sheetId]/maxPoints;
 
  insert /exercise[id]/student[studentId] <result sheet=[sheetId] points=[points] />;
}