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]; }