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