custom variogram models

Hi,

Is it possible to create "custom" variogram models that can be used within the 
gstat framework?

For example, would it be possible to generate a custom exponential model in 
the form of:

gamma = C [ 1 - exp (-3h/a) ]

instead of the typical:

gamma = C [ 1 - exp (-h/a) ]

Thanks,
Dylan

Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341