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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 add_how(voevent, descriptions=None, references=None):
 """Add descriptions or references to the How section. Args: voevent(:class:`Voevent`):
 Root node of a VOEvent etree. descriptions(str):
 Description string, or list of description strings. references(:py:class:`voeventparse.misc.Reference`):
 A reference element (or list thereof). """

    if not voevent.xpath('How'):
        etree.SubElement(voevent, 'How')
    if descriptions is not None:
        for desc in _listify(descriptions):
            # d = etree.SubElement(voevent.How, 'Description')
            # voevent.How.Description[voevent.How.index(d)] = desc
            ##Simpler:
            etree.SubElement(voevent.How, 'Description')
            voevent.How.Description[-1] = desc
    if references is not None:
        voevent.How.extend(_listify(references))