RE: block="substitution"

> There were some WG members, when this design was proposed, who
> ventured to suggest that readers and users might find it a little
> confusing.  But those with more robust faith in the patience and
> ability of the reader to navigate arbitrarily subtle 
> distinctions ended by carrying the day.

This particular reader (of the spec) can follow the byzantine logic. But he
can't work out how to construct an implementation that will generate an
error message that will tell the poor user what on earth is going on.

Fortunately, I have yet to meet a real user who defines substitution groups
and then blocks their use, so perhaps the problem will never arise. 

Or perhaps it will just become one of those half-true cybermyths: "don't use
block, I tried it once and it didn't work".

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