Provide the URL checking tools available in R 4.1+ as a
    package for earlier versions of R. Also uses concurrent requests so
    can be much faster than the serial versions.

R Core team [aut] (The code in urltools.R adapted from the tools
    package),
  Jim Hester [aut] (<https://orcid.org/0000-0002-2739-7082>),
  Gábor Csárdi [aut, cre],
  RStudio [cph, fnd]