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C O S T   F U N C T I O N
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The cost function is a piecewise-linear function that represents the
dependency between the amount of all expenses and the penalty for
spending this money. It must be fully certain and monotonically
increasing, although it may not be strictly increasing.

Intuitively, the cost function converts the amount of spent money into
"penalty points," which are subtracted from the STP schedule score.
We may optionally specify a real-valued weight of the penalty; if we
do not specify it, then it is 1.0. The system multiplies the penalty
by this weight before subtracting it from the schedule score.

We specify the cost function and the related weight of penalty points
in a separate file, which contains no other information.