Add Student ID

addStudentId(ident uid, ident username, ident id) {
  assume exists /account[uid]/admin;
 
  assume exists /account[username]; # implicitly true in OO implementations
  assume not exists /account[username]/student;
  assume count(id, /account/student/id) = 0;
 
  insert /account[username] <student id=[id] />;
}