(Useful for all kinds of upkeep costs)
A switch is a Debt switch have possible values consisting of sets
of Rules-defined currencies (the debt load), with a default of
value of empty (0). Any non-empty debt load is "unpaid", a
empty debt is "paid" or "paid off".
A player CAN flip a specified debt switch by paying Agora a
subset of that switch's debt load for this specified purpose,
thus
removing that subset from that switch's debt load. Attempts
to flip a debt switch by paying a set of currencies that is
not completely in that switch's debt load entirely fail.
[Elsewhere]
Upkeep is a debt switch for each facility tracked by the
Cartographer. At the start of each month, that facility's
Upkeep is set to the facility's upkeep cost defined in the
Rules.
At the end of each month, any facility that is not owned
by Agora and has an unpaid Upkeep is destroyed.
[optionally: reset the debt: when the facility is created?
when the facility changes hands?]
