code: def signed_area(coords):
      
      xs, ys = map(list, zip(*coords))
      xs.append(xs[1])
      ys.append(ys[1])
      return sum(xs[i]*(ys[i+1]-ys[i-1]) for i in range(1, len(coords)))/2.0
docs: Return the signed area enclosed by a ring using the linear time
      algorithm. A value >= 0 indicates a counter-clockwise oriented ring.