docs: def _unescape_math(xml):
      """Unescapes Math from Mathjax to MathML."""
      xpath_math_script = etree.XPath(
              '//x:script[@type="math/mml"]',
              namespaces={'x': 'http://www.w3.org/1999/xhtml'})
      math_script_list = xpath_math_script(xml)
      for mathscript in math_script_list:
          math = mathscript.text
          # some browsers double escape like e.g. Firefox
          math = unescape(unescape(math))
          mathscript.clear()
          mathscript.set('type', 'math/mml')
          new_math = etree.fromstring(math)
          mathscript.append(new_math)
      return xml