Please cite R.oo/R.methodsS3 as
Bengtsson, H. The R.oo package - Object-Oriented Programming with
References Using Standard R Code, Proceedings of the 3rd
International Workshop on Distributed Statistical Computing (DSC
2003), ISSN 1609-395X, Hornik, K.; Leisch, F. & Zeileis, A. (ed.),
2003
A BibTeX entry for LaTeX users is
@InProceedings{,
author = {Henrik Bengtsson},
title = {The {R.oo} package - Object-Oriented Programming with References Using Standard {R} Code},
booktitle = {Proceedings of the 3rd International Workshop on Distributed Statistical Computing (DSC 2003)},
year = {2003},
editor = {Kurt Hornik and Friedrich Leisch and Achim Zeileis},
address = {Vienna, Austria},
month = {March},
issn = {1609-395X},
url = {https://www.r-project.org/conferences/DSC-2003/Proceedings/Bengtsson.pdf},
howpublished = {https://www.r-project.org/conferences/DSC-2003/Proceedings/},
}