code: def add_pathway(self, pathway):
          
          if pathway not in self.pathways:
              self.pathways[pathway] = self.n_pathways
              self.n_pathways += 1
          return self.pathways[pathway]
docs: Updates `self.pathways` and `self.n_pathways.`
  
          Parameters
          -----------
          pathway : str
            the pathway to add to the network.