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