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U T I L I T Y   C A L C
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The Utility Calc algorithm determines the utilities of order changes,
and sorts changes by their utilities. For each change, it computes the
impacts of the change on the schedule score and cost penalty, and
determines the overall utility as the total of these two impacts.

Note that, since the dependency of the cost penalty on the total cost
in nonlinear, the penalty for placing or cancelling a specific order
depends of the current total cost. Thus, the utilities of order
changes depend on the ordering of these changes. When the system
computes utilities of order changes, it assumes that we follow the
suggested ordering when making these changes.


-= Input and output =-

Input: Old order list, new order list, list of order changes, 
  list of resource functions, total cost
Output: Sorted list of order changes with utilities

The input list of order changes should not include utilities; if it
does, the algorithm ignores them. The "total cost" in the input is the
total of all expenses before making any changes; it includes not only
order costs, but also all other conference-related costs.


-= Main steps =-

Set Current-Cost to the given total cost
Create Old-List, which initially includes all order changes
Create New-List, which is initially empty
For every Change in Old-List:
  Compute Score[Change], 
    which is the impact of Change on the schedule score
      Comment: "Score" is positive for placements, negative for
               cancellations, and may be either for modifications
  Compute Cost[Change],
    which is the impact of Change on the total cost
      Comment: "Cost" is positive for placements, negative for
               cancellations, and may be either for modifications      
Repeat while Old-List is nonempty:
  For every Change in Old-List:
    Compute Penalty[Change], 
      which is the impact of Change on the cost penalty,
      assuming that the total cost before Change is Current-Cost
        Comment: "Penalty" is negative for placements, positive for
                 cancellations, any may be either for modifications
    Utility[Change] = Score[Change] + Penalty[Change]
  Select Change with the greatest Utility from Old-List;
    if several changes in Old-List have the same greatest Utility,
    select Change with the smallest Cost among them
      Comment: "Cost" should be the smallest in the math sense;
               for example, -100 is smaller than -50
  Remove Change from Old-List, and add it to the end of New-List
  Current-Cost = Current-Cost + Cost[Change]
Return New-List