* [RDF_GRAPH_THEORY,FIX] rdf_edge/2 would not wrap around rdf_edge/3.
* [RDF_GRAPH_THEORY,FIX] rdf_vertex_check/2 would not skip simple literals that are duplicates of some plain literal in the correct way.
* [GRAPH_SHORT_PATH,GRAPH_TRAVEL,NEW] Split some of the graph travelsal predicates into separate modules. This needs to be cleaned up further.