registerStudent(ident uid, ident id, ident studentId) {
  assume count (studentId, /account/student/id) = 1;
 
  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];
  assume not exists /exercise[id]/student[studentId];
 
  insert /exercise[id] <student id=[studentId] />;
}