code: def extend(self, obj):
          r
          if type(obj) is not list:
              obj = [obj]
          for item in obj:
              if hasattr(item, '_mro'):
                  if 'GenericNetwork' in item._mro():
                      if self.network:
                          raise Exception('Project already has a network')
                  # Must use append since extend breaks the dicts up into
                  # separate objects, while append keeps it as a single object.
                  super().append(item)
              else:
                  raise Exception('Only OpenPNM objects can be added')
docs: r"""
          This function is used to add objects to the project.  Arguments can
          be single OpenPNM objects, an OpenPNM project list, or a plain list of
          OpenPNM objects.