def cleanup_unreachable(rdf):
all_subjects = set(rdf.subjects())<EOL>logging.debug("<STR_LIT>", len(all_subjects))<EOL>reachable = find_reachable(rdf, SKOS.Concept)<EOL>nonreachable = all_subjects - reachable<EOL>logging.debug("<STR_LIT>", len(nonreachable))<EOL>for subj in nonreachable:<EOL><INDENT>delete_uri(rdf, subj)<EOL><DEDENT>
Remove triples which cannot be reached from the concepts by graph
    traversal.