code: def process_order(self, order):
          
          try:
              dt_orders = self._orders_by_modified[order.dt]
          except KeyError:
              self._orders_by_modified[order.dt] = OrderedDict([
                  (order.id, order),
              ])
              self._orders_by_id[order.id] = order
          else:
              self._orders_by_id[order.id] = dt_orders[order.id] = order
              # to preserve the order of the orders by modified date
              move_to_end(dt_orders, order.id, last=True)
  
          move_to_end(self._orders_by_id, order.id, last=True)
docs: Keep track of an order that was placed.
  
          Parameters
          ----------
          order : zp.Order
              The order to record.