code: def update(self, instance, oldValue, newValue):
      
      histogram = self.__get__(instance, None)
      if oldValue:
        histogram[oldValue] -= 1
        if self.autoDelete and histogram[oldValue] == 0:
          del histogram[oldValue]
      if newValue:
        histogram[newValue] += 1
docs: Updates the aggregate based on a change in the child value.