removeStudentId(ident uid, ident username) {
  assume exists //accounts/account[uid]/admin;
 
  assume exists //accounts/account[username]/student; # implies the account exists, which is implicitly true anyway
 
  assume size(//exercises/exercise/students/student[//accounts/account[username]/student/id]) = 0;
  assume size(//exams/exam/results/participant[//accounts/account[username]/student/id]) = 0;
 
  remove //account/account[username]/student;
}