code: def cleanup_unreachable(rdf):
      
  
      all_subjects = set(rdf.subjects())
  
      logging.debug("total subject resources: %d", len(all_subjects))
  
      reachable = find_reachable(rdf, SKOS.Concept)
      nonreachable = all_subjects - reachable
  
      logging.debug("deleting %s non-reachable resources", len(nonreachable))
  
      for subj in nonreachable:
          delete_uri(rdf, subj)
docs: Remove triples which cannot be reached from the concepts by graph
      traversal.