The invisible fence company has come through for us. Apparently, the heap of money we gave them initially covers a full year's worth of unconditional service. So snow-blowers, plows, well-diggers, comets, and other unforeseen causes of fence destruction qualify for free repair.
Not too shabby, really.
My only question now is: How are they going to repair a wire that has been plowed into a snowbank? Especially now that a day of rain has melted the snowbank from a fluffy pile of semi-moveable snow into a condensed ice-prison.