docs: def sub_nav(context):
      """
      Sub-nav displayed below proposition header
      - creates alternate language links if SHOW_LANGUAGE_SWITCH is set
      - takes "breadcrumbs" from the context
      - takes "breadcrumbs_back" from the context to show a back link *instead* of breadcrumbs
      """
      request = context.get('request')
      return {
          'alt_urls': make_alternate_language_urls(request),
          'breadcrumbs': context.get('breadcrumbs'),
          'breadcrumbs_back': context.get('breadcrumbs_back'),
      }