anchor.diff:  
   %% @doc When asked for properties of an RDF resource, ask the RDF data source to
   %%      provide extra properties
  +observe_rsc_get(#rsc_get{}, [], _Context) ->
  +    [];
   observe_rsc_get(#rsc_get{id=_Id}, Props, Context) ->
   
       %% Only act on non-authoritative resources
  
positive.code:     z_datamodel:manage(?MODULE, Datamodel, Context),
  
      %% Update some predicates so they can refer to category RDF, too
      case m_rsc:uri_lookup("http://xmlns.com/foaf/0.1/depiction", Context) of 
          undefined -> noop;
          PredId ->
              Objects = m_predicate:objects(PredId, Context),
  
negative.code:         false ->
              RscUri = proplists:get_value(uri, Props),
              case z_utils:is_empty(RscUri) of
                  true ->
                      Props;
                  false ->
                      z_notifier:foldl(#rdf_get{uri=RscUri}, Props, Context)
  