new_contents: -__author__= 'Dan and Pat'
  -#Describes an Observable object. 
  -#includes methods to notify all observers
  -#and to add/delete them
  -
  -#abstract class for Observable
  -class Observable(object):
  -
  -    #list of all observers
  -    observer_list = []
  -
  -    #notify all observers
  -    def notify_all(self, msg):
  -        for obs in self.observer_list:
  -            obs.notify(msg)
  -
  -    #add observer to the list
  -    def add_observer(self, observer):
  -        if observer not in self.observer_list:
  -            self.observer_list.append(observer)
  - 
  -    #delete all observers
  -    def delete_all_observers(self):
  -        del self.observer_list[:]
  -    
  
subject: Create Observable feature