Buses roll toward Pinkerton before most people have finished a first cup of coffee, which means the driveways along the way have to be open earlier than that. Mr. B's Roofing runs snow in Derry with that clock in mind — plowing, shoveling walkways and entries, clearing around parked cars, sanding wherever a driveway tips toward the street. Nor'easters arrive loud, but the tricky ones are the small overnight squalls that leave four slick inches nobody planned for. The truck goes out for both. First tracks in fresh snow are a fine thing to leave behind, and better still to erase completely.