docs: def get_plural_tag_index(number, locale):
      """Gets the plural tag index of a number on the plural rule of a locale::
  
         >>> get_plural_tag_index(1, 'en_US')
         0
         >>> get_plural_tag_index(2, 'en_US')
         1
         >>> get_plural_tag_index(100, 'en_US')
         1
  
      """
      locale = Locale.parse(locale)
      plural_rule = locale.plural_form
      used_tags = plural_rule.tags | set([_fallback_tag])
      tag, index = plural_rule(number), 0
      for _tag in _plural_tags:
          if _tag == tag:
              return index
          if _tag in used_tags:
              index += 1