This interface provides access to a collection of global variables. This abstraction is used by the optimizer
to handle the rather different ways that global variables are managed in XSLT and XQuery, as a result of
Get an existing global variable whose initializer is a given expression,
if there is one. Note that this depends on the logic for detecting equivalence
of expressions, which is necessarily approximate. Expressions with dependencies
on the static context should never be considered equivalent. If no equivalent global
variable is found, return null. An implementation can always return null if
it wants to avoid a lengthy search.
select - the expression to which the variable is bound
an existing global variable with the same select expression, if one
can be found; otherwise null.