The declare context item declaration in the query prolog is implemented. This allows a
required type and a default value to be declared for the context item.
At present (the rules aren't entirely clear) it is possible to specify a value from the calling API, or to not specify a value, regardless whether "external" is specified or not.
At present there is no interaction with the API facilities for defining a required type for the context item: both can be used independently.
