public static double Average_Cell_StraightLineVisibility(this PM_Maze maze)
        {
            int width  = maze.Q_Width();
            int height = maze.Q_Height();

            double sum = 0.0;

            for (int x = 0; x < width; x++)
            {
                for (int y = 0; y < height; y++)
                {
                    sum += (double)maze.Cell__StraightLineVisibility(x, y);
                }
            }

            int numCells = maze.NumCells__All();

            return(sum / (double)numCells);
        }