Unregister Student
unregisterStudent(ident uid, ident id, ident studentId) {
assume exists /exercise[id] && /exercise[id]/open &&
exists /account[uid]/student && /account[uid]/student/id = studentId
|| exists /account[uid]/assistant[id];
assume exists /exercise[id]/student[studentId]; # implies the exercise exists
assume size(/exercise[id]/student[studentId]/result) = 0; # don't allow unregistering if results are there
assume not exists /exercise[id]/student[studentId]/group; # don't allow if signed in a group
remove /exercise[id]/student[studentId];
}