QueryExpression type

[David Hull <dmh@tibco.com>]

After re-reading in more detail, I believe one of my previous comments 
on precondition and selector was slightly off.

These elements are defined as having type QueryExpression, which is just 
a dialect attribute and mixed content.  I had mistakenly assumed they 
contained an element of type QueryExpression.  As the sample message 
shows, the precondition element in the subscribe message already looks 
like I suggested it should.

The main difference between Precondition and Selector on the one hand 
and TopicExpression on the other is that TopicExpression has type 
wsnt:TopicExpressionType while the others have type 
wsrp:QueryExpressionType.  The two types have essentially identical 
definitions.

In either case, we want to be able to allow any kind of content but 
easily and explicitly know what sort of content it is.   It seems odd 
that we have two different definitions of "arbitrary content tagged with 
a dialect ID".  If this concept is useful, it seems more generally 
useful than either topic expression or query expression.  An alternative 
would be for all three elements to have the same type, and for that type 
to have a more generic name than "QueryExpression".