> given that the operations permitted on a result tree fragment are a
> subset of those permitted on a node-set?

actually the operations permitted on an rtf are exactly those allowed on
string. So it all depends how the OP wants to model null, just use a
value to model null that's not used elsewhere and then test for that.
 "" 
or 
"null"
or 
"why any data outside of an SQL database wants a null value baffles me"

David

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