docs: def _rank_tags(tagged_paired_aligns):
      '''Return the list of tags ranked from most to least popular.'''
      tag_count_dict = defaultdict(int)
      for paired_align in tagged_paired_aligns:
          tag_count_dict[paired_align.umt] += 1
      tags_by_count = utils.sort_dict(tag_count_dict)
      ranked_tags = [tag_count[0] for tag_count in tags_by_count]
      return ranked_tags