code: def extract_images_jbig2(pike, root, log, options):
      
  
      jbig2_groups = defaultdict(list)
      for pageno, xref, ext in extract_images(
          pike, root, log, options, extract_image_jbig2
      ):
          group = pageno // options.jbig2_page_group_size
          jbig2_groups[group].append((xref, ext))
  
      # Elide empty groups
      jbig2_groups = {
          group: xrefs for group, xrefs in jbig2_groups.items() if len(xrefs) > 0
      }
      log.debug("Optimizable images: JBIG2 groups: %s", (len(jbig2_groups),))
      return jbig2_groups
docs: Extract any bitonal image that we think we can improve as JBIG2