revokeTutorRights(ident uid, ident username, ident exerciseId, ident groupId) {
  assume exists /account[uid]/assistant[exerciseId];
 
  assume exists /account[username]/tutor[exerciseId]/group[groupId];
  assume exists /exercise[exerciseId]/group[groupId]/tutor[username];
    # implied by integrity and the assumption before
 
  remove /account[username]/tutor[exerciseId]/group[groupId];
  if size(/account[username]/tutor[exerciseId]/group) = 0 then # should this be an additional procedure?
    remove /account[username]/tutor[exerciseId];
  fi
  remove /exercise[exerciseId]/group[groupId]/tutor[username];
}