BindingReference is a interface used to mark references to a variable declaration. The main
implementation is VariableReference, which represents a reference to a variable in an XPath
expression, but it is also used to represent a reference to a variable in a saxon:assign instruction.
Fix up the static type of this variable reference; optionally, supply a constant value for
the variable. Also supplies other static properties of the expression to which the variable
is bound, for example whether it is an ordered node-set.
type - The static type of the variable reference, typically either the declared type
of the variable, or the static type of the expression to which the variable is bound
constantValue - if non-null, indicates that the value of the variable is known at compile
time, and supplies the value
properties - static properties of the expression to which the variable is bound