Spare Budget for a Party
There may be an amount of spare budget, which may be expressed explicitly in any object with an optional extra key.
Every object that performs accounting may be setup with a spare budget by adding
an extra key "spare" with the non-negative numeric value. This value is never
obliged, as the accounting rules accept any non-negative budgetting structure.
If used, the "spare" key can be added to any party object, or to the overall
object, to index a values array with any budget that is currently neither used
nor planned, not even as an uncertainty marin.