addParticipant(ident uid, ident id, ident studentId) {
  assume exists //exams/exam[id]/free && exists //accounts/account[uid]/student 
             && //accounts/account[uid]/student/id = studentId
      || exists //accounts/account[uid]/examiner[id];
 
  assume exists //exams/exam[id]; # implicitly true in OO implementations
  assume exists //accounts/account/student[studentId];
 
  assume not exists //exams/exam[id]/results/participant[studentId];
 
  insert //exams/exam[id]/results/participant[studentId];
}