Protein subclass

hi all,

I am trying to define a subclass named Protein, but this doesn't work 
with any tested xace versions (current supported or daily)
Is this forbidden? If yes, shouldn't xace/tace complain when it reads 
the models?

Here is a small example to illustrate:

######################################
$ more models.wrm
?Ligand Protein
         Sugar

######################################
$ more subclasses.wrm
Class Prot
Visible
Is_a_subclass_of Ligand
Filter Protein

Class Protein
Visible
Is_a_subclass_of Ligand
Filter Protein

Class Sugar
Visible
Is_a_subclass_of Ligand
Filter Sugar