+/** Declare the xref_value corresponding to the given key as being used as the
+ given type. The use of this function is not mandatory, but it can aid in
+ spotting places in the code where xref_value objects are deleted while
+ they are still referenced.
+
+ \param type The type of the referenced object
+ \param xrefstr The key for the object
+
+ \return The xref_value object if success, and its use count is incremented.
+ Returns NULL in one of the following cases:
+ - the key isn't a valid cross-reference key (see detailed description of
+ \ref parsed_xref)
+ - there is no xref_value with the given key
+ - the xref_value was previously added as another type than the type
+ provided here
+ */