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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 html_to_cnxml(html_source, cnxml_source):
 """Transform the HTML to CNXML. We need the original CNXML content in order to preserve the metadata in the CNXML document. """

    source = _string2io(html_source)
    xml = etree.parse(source)
    cnxml = etree.parse(_string2io(cnxml_source))
    # Run the HTML to CNXML transform on it
    xml = _transform('html5-to-cnxml.xsl', xml)
    # Replace the original content element with the transformed one.
    namespaces = {'c': 'http://cnx.rice.edu/cnxml'}
    xpath = etree.XPath('//c:content', namespaces=namespaces)
    replaceable_node = xpath(cnxml)[0]
    replaceable_node.getparent().replace(replaceable_node, xml.getroot())
    # Set the content into the existing cnxml source
    return etree.tostring(cnxml)