The name of the external variable, expressed
as a QName. If an external variable of this name has been declared in the
query prolog, the given value will be assigned to the variable. If the
variable has not been declared, calling this method has no effect (it is
not an error).
value
The value to be given to the external variable.
If the variable declaration defines a required type for the variable, then
this value must match the required type: no conversions are applied.