x, y = 0, 0
  xmin, xmax, ymin, ymax = 0, 0, 0, 0
  e = Enumerator.new do |yy|
    loop do
      while x > xmin
        yy << [x, y]
        x -= 1
      end
      xmin -= 1
      while y < ymax
        yy << [x, y]
        y += 1
      end
      ymax += 1
      while x < xmax
        yy << [x, y]
        x += 1
      end
      xmax += 1
      while y > ymin
        yy << [x, y]
        y -= 1
      end
      ymin -= 1
    end
  end