code: def merge_output(self, result, items, template_name):
          
          html_output, media = self.get_html_output(result, items)
  
          if not template_name:
              merged_html = mark_safe(u''.join(html_output))
          else:
              context = {
                  'contentitems': list(zip(items, html_output)),
                  'parent_object': result.parent_object,  # Can be None
                  'edit_mode': self.edit_mode,
              }
  
              context = PluginContext(self.request, context)
              merged_html = render_to_string(template_name, context.flatten())
  
          return ContentItemOutput(merged_html, media, cacheable=result.all_cacheable, cache_timeout=result.all_timeout)
docs: Combine all rendered items. Allow rendering the items with a template,
          to inserting separators or nice start/end code.