removeResult(ident uid, ident id, ident studentId, ident sheetId) {
  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]/result[sheetId]; # implies the existence of pretty much everything
 
  remove //exercises/exercise[id]/students/student[studentId]/result[sheetId];
}