R Under development (unstable) (2019-08-23 r77061) -- "Unsuffered Consequences"
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> library(survival)
> 
> # the way a +cluster() term is handled in coxph has implications for update.
> 
> fit1 <- coxph(Surv(time, status) ~ age, cluster= inst, lung)
> fit2 <- coxph(Surv(time, status) ~ age + cluster(inst), lung)
> all.equal(fit1, fit2)
[1] TRUE
> 
> fit3 <- coxph(Surv(time, status) ~ age + sex + cluster(inst), lung)
> 
> test1 <- update(fit1, .~ .+ sex)
> all.equal(test1, fit3)
[1] TRUE
> 
> # Gives a spurious warning message
> test2 <- update(fit1, . ~ age + sex + cluster(inst), lung)
Warning message:
In coxph(formula = Surv(time, status) ~ age + sex + cluster(inst),  :
  cluster appears both in a formula and as an argument, formula term ignored
> all.equal(test2, fit3)
[1] TRUE
> 
> 
> proc.time()
   user  system elapsed 
  0.751   0.039   0.791