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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_feed_list(opml_obj: OPML) -> List[str]: """Walk an OPML document to extract the list of feed it contains."""

    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