3.4 Issues with rdf_db
This RDF low-level module has been created after two year 
experimenting with a plain Prolog based module and a brief evaluation of 
a second generation pure Prolog implementation. The aim was to be able 
to handle upto about 5 million triples on standard (notebook) hardware 
and deal efficiently with subPropertyOf which was 
identified as a crucial feature of RDFS to realise fusion of different 
data-sets.
The following issues are identified and not solved in suitable manner.
subPropertyOfofsubPropertyOf- is not supported.
 - Equivalence
 - Similar to 
subPropertyOf, it is likely to be profitable to handle resource identity efficient. The current system has no support for it.