[R-sig-Geo] Cubic variogram models - in vgm( )? elsewhere?

[Ted]

Hi all -

Apologies if the answer to this is obvious - it isn't to me so any help appreciated.

I have been asked to match in R some kriging done in Isatis (BTW, this is not part
of any assessment task). The model that was used in Isatis was a cubic one. Specifically,
it is a nested cubic one of the form:
  
  Structure 1: Cubic(60', 80') sill = 220
  Structure 2: Cubic(Inf, 80') sill = 100

That's probably more information than needed for my question. Basically it is clear that 
we are dealing with zonal anisotropy (managed by extreme geometric anisotropy). 

But how do I generate a cubic model in R? I have been using vgm() and although cubic
models are not commonly used I have this challenge placed before me and wonder what
I can do in the absence of an explicit "Cub" model  :-)

I'm not even sure that this is a reasonable request but any guidance gratefully received.

TP