# CLUSTER-ROBUST STANDARD ERRORS FOR LINEAR MODELS BY MAHMOOD ARIAS
clx <- function(fm, dfcw, cluster){
    M <- length(unique(cluster))
    N <- length(cluster)
    dfc <- (M/(M-1))*((N-1)/(N-fm$rank))
    u  <- apply(estfun(fm),2,
    function(x) tapply(x, cluster, sum))
    vcovCL <- dfc*sandwich(fm, meat=crossprod(u)/N)*dfcw
    coeftest(fm, vcovCL) 
}