old_contents: class Exchange(object):
      def get_markets(self):
          """
          Returns a list of tuples of the form ('XXX', 'YYY') representing the 
          available markets on the exchange, where XXX and YYY are the currency 
          codes for the base currency and counter currency, respectively.
          """
          raise NotImplementedError
  
      def get_my_open_orders(self):
          """
          Returns a list of exchanges.order.Order that represent currently 
          unfulfilled trade orders.
          """
          raise NotImplementedError
  
      def get_my_trades(self):
          """
          Returns a list of exchanges.trade.Trade that represent all the user's 
          trades.
          """
          raise NotImplementedError
  
      def cancel_order(self, order_id):
          """
          Given an order_id, cancels the order associeted with the id. Returns 
          None.
          """
          raise NotImplementedError
      
      def buy(self, frm, to, quantity, price):
          raise NotImplementedError
  
      def sell(self, frm, to, quantity, price):
          raise NotImplementedError
  
new_contents: class Exchange(object):
      def get_markets(self):
          """
          Returns a list of tuples of the form ('XXX', 'YYY') representing the 
          available markets on the exchange, where XXX and YYY are the currency 
          codes for the base currency and counter currency, respectively.
          """
          raise NotImplementedError
  
      def get_my_open_orders(self):
          """
          Returns a list of exchanges.order.Order that represent currently 
          unfulfilled trade orders.
          """
          raise NotImplementedError
  
      def get_my_trades(self):
          """
          Returns a list of exchanges.trade.Trade that represent all the user's 
          trades.
          """
          raise NotImplementedError
  
      def cancel_order(self, order_id):
          """
          Given an order_id, cancels the order associeted with the id. Returns 
          None.
          """
          raise NotImplementedError
      
      def buy(self, market, quantity, price):
          raise NotImplementedError
  
      def sell(self, market, quantity, price):
          raise NotImplementedError
  
subject: Modify buy and sell interface to take market tuples