library(mets)
n <- 10;
d <- mets:::simCox(n); d$id <- seq(nrow(d)); d$group <- factor(rbinom(nrow(d),1,0.5))
m1 <- phreg(Surv(entry, time,status)~X1+X2+cluster(id)+strata(group),data=d)
riskRegression:::model.matrix(m1, d)
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