Zeileis A (2006).
“Object-Oriented Computation of Sandwich Estimators.”
*Journal of Statistical Software*, **16**(9), 1--16.
tools:::Rd_expr_doi("10.18637/jss.v016.i09")

Zeileis A, Köll S, Graham N (2020).
“Various Versatile Variances: An Object-Oriented Implementation of Clustered Covariances in R.”
*Journal of Statistical Software*, **95**(1), 1--36.
tools:::Rd_expr_doi("10.18637/jss.v095.i01")