Issue: Should we disallow nested grammars?

[Murata Makoto <mura034@attglobal.net>]

At present, nesting of grammars is freely allowed.  I propose 
to disallow such nesting in V1.0.

First, I do not see any use cases.  The TREX tutorial says:
"Of course, in a trivial case like this, there is no advantage in nesting 
the grammars: we could simply have have included table.trex within the outer 
grammar element."

It further says: "However, when the included grammar has many definitions, 
nesting it avoids the possibility of name conflicts between the including 
grammar and the included grammar."  In other words, we only have to perform 
renaming upon inclusion.  This is very easy.

Second, I believe that modularization mechanisms requires further study.  
I think that it is dangerous to provide nested grammars without fully 
understanding modularization.

Cheers,

Makoto