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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 print_model(self, pretty=True, encoding='utf8'):
 """Return the assembled SBGN model as an XML string. Parameters pretty : Optional[bool] If True, the SBGN string is formatted with indentation (for human viewing) otherwise no indentation is used. Default: True Returns ------- sbgn_str : bytes (str in Python 2) An XML string representation of the SBGN model. """

        return lxml.etree.tostring(self.sbgn, pretty_print=pretty,
                                   encoding=encoding, xml_declaration=True)