revokeExaminerRights(ident uid, ident username, ident examId) {
  assume exists /account[uid]/admin;
 
  assume exists /account[username]/examiner[examId]; # implies the account exists, which is implicitly true anyway
  assume exists /exam[examId]/examiner[username]; # implied by integrity and the assumption before
 
  remove /account[username]/examiner[examId];
  remove /exam[examId]/examiner[username];
}