Binding is a interface used to represent the run-time properties and methods
associated with a variable: specifically, a method to get the value
of the variable.
Test whether it is permitted to assign to the variable using the saxon:assign
extension element. This will only be for an XSLT global variable where the extra
attribute saxon:assignable="yes" is present.
Indicate whether the binding is local or global. A global binding is one that has a fixed
value for the life of a query or transformation; any other binding is local.