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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 tostring(self, fully_qualified=True, pretty_print=True, encoding="UTF-8"):
 """ Serialize and return a string of this METS document. To write to file, see :meth:`write`. The default encoding is ``UTF-8``. This method will return a unicode string when ``encoding`` is set to ``unicode``. :return: String of this document """

        root = self.serialize(fully_qualified=fully_qualified)
        kwargs = {"pretty_print": pretty_print, "encoding": encoding}
        if encoding != "unicode":
            kwargs["xml_declaration"] = True
        return etree.tostring(root, **kwargs)