[R] How do I specify a partially completed survival analysis model?

[RWilliam]

Hello,

I just started using R to do epidemiologic simulation research using the Cox
proportional hazard model. I have 2 covariates X1 and X2 which I want to
model as h(t,X)=h0(t)*exp(b1*X1+b2*X2). I assume independence of X from t. 

After I simulate Time and Censor data vectors denoting the censoring time
and status respectively, I can call the following function to fit the data
into the Cox model (a is a data.frame containing 4 columns X1, X2, Time and
Censor):
b = coxph (Surv (Time, Censor) ~ X1 + X2, data = a, method = "breslow");

Now the purpose of me doing simulation is that I have another mechanism to
generate the number b2. From the given b2 (say it's 4.3), Cox model can be
fit to generate b1 and check how feasible the new model is. Thus, my
question is, how do I specify such a model that is partially completed (as
in b2 is known). I tried things like Surv(Time,Censor)~X1+4.3*X2, but it's
not working. Thanks very much.
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