code: def write_backreferences(seen_backrefs, gallery_conf,
                           target_dir, fname, snippet):
      
      if gallery_conf['backreferences_dir'] is None:
          return
  
      example_file = os.path.join(target_dir, fname)
      backrefs = scan_used_functions(example_file, gallery_conf)
      for backref in backrefs:
          include_path = os.path.join(gallery_conf['src_dir'],
                                      gallery_conf['backreferences_dir'],
                                      '%s.examples.new' % backref)
          seen = backref in seen_backrefs
          with codecs.open(include_path, 'a' if seen else 'w',
                           encoding='utf-8') as ex_file:
              if not seen:
                  heading = '\n\nExamples using ``%s``' % backref
                  ex_file.write(heading + '\n')
                  ex_file.write('^' * len(heading) + '\n')
              ex_file.write(_thumbnail_div(target_dir, gallery_conf['src_dir'],
                                           fname, snippet, is_backref=True))
              seen_backrefs.add(backref)
docs: Writes down back reference files, which include a thumbnail list
      of examples using a certain module