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