nested variograms?

A quick question for experienced gstat users....
Can nested variograms be fit using gstat? if so, is it simply adding 
to an existing variogram structure?

Thanks

> library(gstat)
Loading required package: sp
 > data(meuse)
 > coordinates(meuse) = c("x", "y")
 > v = variogram(log(zinc) ~ 1, meuse)
 > fit.variogram(v, vgm(1, "Exp", 300))
  model     psill    range
1   Exp 0.7186595 449.7664
 > fit.variogram(v, vgm(1, "Exp", 300, 1))
  model     psill    range
1   Nug 0.0000000   0.0000
2   Exp 0.7186528 449.7583
 > fit.variogram(v, vgm(1, "Exp", 300, add.to=vgm(1, "Nug", 0)))
  model     psill    range
1   Nug 0.0000000   0.0000
2   Exp 0.7186528 449.7583
 > fit.variogram(v, vgm(1, "Exp", 300, 1, add.to=vgm(1, "Exp", 10)))
Warning: singular model in variogram fit
  model psill range
1   Exp     1    10
2   Nug     1     0
3   Exp     1   300
 > fit.variogram(v, vgm(1, "Exp", 300, 1, add.to=vgm(1, "Exp", 10, 
add.to=vgm(1, "Exp", 50))))
Warning: singular model in variogram fit
  model psill range
1   Exp     1    50
2   Exp     1    10
3   Nug     1     0
4   Exp     1   300

Of course the fitting fails badly for this poor example, but I guess you 
get the idea. I think there is no limit to the number of nested 
structures, but consider that feature as pretty useless.

Edzer Pebesma
Institute for Geoinformatics (ifgi), University of M?nster,
Weseler Stra?e 253, 48151 M?nster, Germany.  Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de/