To cite package ‘proto’ in publications use:

  Grothendieck G, Kates L, Petzoldt T (2016). _proto: Prototype
  Object-Based Programming_. R package version 1.0.0.9000,
  <https://github.com/hadley/proto>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {proto: Prototype Object-Based Programming},
    author = {Gabor Grothendieck and Louis Kates and Thomas Petzoldt},
    year = {2016},
    note = {R package version 1.0.0.9000},
    url = {https://github.com/hadley/proto},
  }