grantTutorRights(ident uid, ident username, ident exerciseId, ident groupId) {
  assume exists /account[uid]/assistant[exerciseId];
 
  assume exists /account[username]; # implicitly true in OO implementations
  assume exists /exercise[exerciseId]/group[groupId];
 
  assume not exists /account[username]/tutor[exerciseId]/group[groupId]);
  assume not exists /exercise[exerciseId]/group[groupId]/tutor[username];
    # implied by integrity and the assumption before
 
  if not exists /account[username]/tutor[exerciseId] then # we could split the tutor rights here
    insert /account[username] <tutor exercise=[exerciseId] />;
  fi
  insert /account[username]/tutor[exerciseId] <group id=[groupId] />;
  insert /exercise[exerciseId]/group[groupId] <tutor account=[account] />;
}