removeParticipant(ident uid, ident id, ident studentId) {
  assume exists //accounts/account[uid]/examiner[id];
 
  assume exists //exams/exam[id]/results/participant[studentId]; # implies the exam exists, which is implicitly true anyway
 
  assume size(//exams/exam[id]/results/participant[studentId]/result) = 0; # don't allow to remove if results are there
 
  remove //exams/exam[id]/results/participant[studentId];
}