code: def get_feed_list(opml_obj: OPML) -> List[str]:
      
      rv = list()
  
      def collect(obj):
          for outline in obj.outlines:
              if outline.type == 'rss' and outline.xml_url:
                  rv.append(outline.xml_url)
  
              if outline.outlines:
                  collect(outline)
  
      collect(opml_obj)
      return rv
docs: Walk an OPML document to extract the list of feed it contains.