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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 _tree_to_string(cls, root_element, xml_declaration=True, pretty_print=True):
 """ Return an ``lxml`` tree as a Unicode string. """

        from lxml import etree
        return gf.safe_unicode(etree.tostring(
            root_element,
            encoding="UTF-8",
            method="xml",
            xml_declaration=xml_declaration,
            pretty_print=pretty_print
        ))