This is the R package `crs' (Categorical Regression Splines) written and maintained by Jeffrey S. Racine (firstname.lastname@example.org) with the invaluable assistance of Zhenghua Nie.
You can install the stable version on CRAN:
install.packages('crs', dependencies = TRUE)
library(devtools); install_github('R-Package-crs', 'JeffreyRacine')
Note Windows users have to first install Rtools, while OS X users have to first install Xcode and the command line tools (in OS X 10.9 or higher, once you have Xcode installed, open a terminal and run xcode-select --install).
For more information on this project please visit the maintainer's website (https://www.socialsciences.mcmaster.ca/people/racinej).