R Under development (unstable) (2023-08-12 r84939 ucrt) -- "Unsuffered Consequences"
Copyright (C) 2023 The R Foundation for Statistical Computing
Platform: x86_64-w64-mingw32/x64

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.

> 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 
   0.82    0.15    0.96