docs: def _add_bound_method(self, bound_method, identify_observed):
          """Add an bound method as an observer.
  
          Args:
              bound_method: The bound method to add as an observer.
              identify_observed: See the docstring for add_observer.
  
          Returns:
              True if the bound method is added, otherwise False.
          """
  
          inst = bound_method.__self__
          method_name = bound_method.__name__
          key = self.make_key(bound_method)
          if key not in self.observers:
              self.observers[key] = ObserverBoundMethod(
                  inst, method_name, identify_observed, (key, self.observers))
              return True
          else:
              return False