To cite package ‘vcdExtra’ in publications use:

  Friendly M (2023). _vcdExtra: 'vcd' Extensions and Additions_. R
  package version 0.8-5, <https://github.com/friendly/vcdextra>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {vcdExtra: 'vcd' Extensions and Additions},
    author = {Michael Friendly},
    year = {2023},
    note = {R package version 0.8-5},
    url = {https://github.com/friendly/vcdextra},
  }