[R] Nested frailty model

[ah06981 at studbocconi.it]

Dear R-help,
I am trying to estimate a Cox model with nested effects, or better

h(t,v,w)=v*w*h0(t)*exp(B'x)

where h(t,v,w) is the individual hazard function
       w and v are both frailty terms (gamma or normal distributed)
I have 12 clusters and for each one of them I would like to associate a 
realization of v, while w is a random effect for the whole population. 
At the population level I should integrate out both the frailty 
distributions.
This type of model is usually called nested frailty model.
I would like to know if there is a package which could allow me to 
estimate it.

Thank you very much indeed
Silvia