removeResult(ident uid, ident id, ident studentId, ident sheetId) {
  assume exists /account[uid]/assistant[id]
      || exists /account[uid]/tutor[id]/group[/exercise[id]/student[studentId]/group];
 
  assume exists /exercise[id]/student[studentId]/result[sheetId]; # implies the existence of pretty much everything
 
  remove /exercise[id]/student[studentId]/result[sheetId];
}