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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 toc_html(self, depth=6, lowest_level=6):
 """ Get TOC of currently fed HTML string in form of HTML string. :param depth: the depth of TOC :param lowest_level: the allowed lowest level of header tag :return: an HTML string """

        toc = self.toc(depth=depth, lowest_level=lowest_level)
        if not toc:
            return ''

        def map_toc_list(toc_list):
            result = ''
            if toc_list:
                result += '<ul>\n'
                result += ''.join(
                    map(lambda x: '<li>'
                                  '<a href="#{}">{}</a>{}'
                                  '</li>\n'.format(
                        x['id'], x['inner_html'],
                        map_toc_list(x['children'])),
                        toc_list)
                )
                result += '</ul>'
            return result

        return map_toc_list(toc)