[R] Bootstrap of Kaplan-Meier Estimate for clustered data

[M. Posch]

Is there an R-procedure to calculate a Kaplan Meier estimate with 
confidence bounds for correlated survival times (with correlated groups of 
observations  which can be accounted for with the "cluster" argument in the 
cox regression). I would need it to estimate the survival time of dental 
implants, where each patient has several implants.

Thanks,

Martin