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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, include_unsigned_edges=False):
 """Return a SIF string of the assembled model. Parameters include_unsigned_edges : bool If True, includes edges with an unknown activating/inactivating relationship (e.g., most PTMs). Default is False. """

        sif_str = ''
        for edge in self.graph.edges(data=True):
            n1 = edge[0]
            n2 = edge[1]
            data = edge[2]
            polarity = data.get('polarity')
            if polarity == 'negative':
                rel = '-1'
            elif polarity == 'positive':
                rel = '1'
            elif include_unsigned_edges:
                rel = '0'
            else:
                continue
            sif_str += '%s %s %s\n' % (n1, rel, n2)
        return sif_str