This is a "smart" variable reference that is used in situations where
an absolute path is optimized into a variable reference, but may
be used in some situations where the document context may have changed.
For instance, in select="document(doc/@href)//name[//salary > 7250]", the
root in the predicate will be different for each node in the set. While
this is easy to detect statically in this case, in other cases static
detection would be very hard or impossible. So, this class does a dynamic check
to make sure the document context of the referenced variable is the same as
the current document context, and, if it is not, execute the referenced variable's
expression with the current context instead.
Dereference the variable, and return the reference value. Note that lazy
evaluation will occur. If a variable within scope is not found, a warning
will be sent to the error listener, and an empty nodeset will be returned.