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O R D E R    C H A N G E S
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A list of "order changes" represents the difference between an old
list of orders and a new list. It includes additions of new orders,
and changes and deletions of old orders, required for converting an
old list into a new list. It consists of the following three parts.

() Placements: The list of new orders that we need to place; the
specification of each order includes the related event id and service
property. Note that this specification uniquely defines an order, and
enables the system to look up this order in the list of new orders.

() Cancellations: The list of old orders that we need to cancel,
specified by the respective event ids and service properties. The
system has to look up these orders in the list of old orders.

() Modifications: The list of order pairs, where each pair includes an
old order and a new order.

For each placement, cancellation, and modification, the list may
optionally include its utility, which is the expected score increase
due to making this change. The list may include utilities for some
elements, and no utilities for the other elements. The system uses
utilities to prioritize the related tasks for the user.

We store a list of order changes in a separate file, which includes a
reference to the related lists of old orders and new orders. Note that
we cannot use the list of changes without the related order lists.