RE: choice attributeGroup

> This is not my idea. It is very old format of XML and now I 
> have to prepare schema for it.

Unfortunately, there are many constraints that XML Schema can't express, so
if you design your documents first and then try to write a schema for them,
you often run into this. (Note that DTDs can't handle this either).

> I think, that is impossible because XML schema recognize only 
> 'context-free grammar'.

In this case it's not so much that the grammar is context free, as the fact
that the tokens that take the grammar from one state to another are element
names, not attribute values.

Michael Kay
http://www.saxonica.com/