docs: def discard_observer(self, observer):
          """Un-register an observer.
  
          Args:
              observer: The observer to un-register.
  
          Returns true if an observer was removed, otherwise False.
          """
          discarded = False
          key = self.make_key(observer)
          if key in self.observers:
              del self.observers[key]
              discarded = True
          return discarded