To cite package ‘lookup’ in publications use:

Hester J, Wickham H, Csárdi G (2019). lookup: Lookup R function definitions, including compiled code, S3 and S4 methods. R package version 0.0.0.9000, https://github.com/jimhester/lookup.

Corresponding BibTeX entry:

  @Manual{,
    title = {lookup: Lookup R function definitions, including compiled
      code, S3 and S4 methods},
    author = {Jim Hester and Hadley Wickham and Gábor Csárdi},
    year = {2019},
    note = {R package version 0.0.0.9000},
    url = {https://github.com/jimhester/lookup},
  }