optimization for IOmemory's len function
optimization for IOmemory's len function

no need to iterate over the set anymore.

marma/rdflib,dbs/rdflib,armandobs14/rdflib,yingerj/rdflib,avorio/rdflib,RDFLib/rdflib,avorio/rdflib,dbs/rdflib,ssssam/rdflib,dbs/rdflib,yingerj/rdflib,RDFLib/rdflib,yingerj/rdflib,armandobs14/rdflib,yingerj/rdflib,marma/rdflib,avorio/rdflib,marma/rdflib,ssssam/rdflib,dbs/rdflib,avorio/rdflib,armandobs14/rdflib,ssssam/rdflib,RDFLib/rdflib,ssssam/rdflib,RDFLib/rdflib,marma/rdflib,armandobs14/rdflib