Three towns, A, B, and C, have a funny arrangement with respect
to the power grid. They all get their power from a single hydraulic
power plant up the river. Both cities A and B each have power lines
that supply power directly from the plant. Furthermore, there is a
supply line between A and B, so that either city will not lose
power in the case that one of their direct lines from the plant
fails. (Of course, if...