docs: def _add_function(self, func, identify_observed):
          """Add a function as an observer.
  
          Args:
              func: The function to register as an observer.
              identify_observed: See docstring for add_observer.
  
          Returns:
              True if the function is added, otherwise False.
          """
  
          key = self.make_key(func)
          if key not in self.observers:
              self.observers[key] = ObserverFunction(
                  func, identify_observed, (key, self.observers))
              return True
          else:
              return False