Zeileis, Achim, Susanne Köll, and Nathaniel Graham. “Various Versatile Variances: An Object-Oriented Implementation of Clustered Covariances in R”. Journal of Statistical Software 95, no. 1 (October 7, 2020): 1–36. Accessed February 3, 2023. https://www.jstatsoft.org/index.php/jss/article/view/v095i01.