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>system("g.list rast") shows my raster maps properly.
Then I've tried to follow the steps indicated in Bivand, (2000). And typed
>tapply(dem, cover,summary)
(where dem is a Digital Elevation Model, and cover is a map of different 
clases of vegetal cover)
and the following message appears:
 Object "cover" not found.
  
I've tried the same with different location and mapsets and none works.


Am I missing something?
Any previous step is necessary?

Javier Garcia