code: def print_model(self, include_unsigned_edges=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
docs: 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.