docs: def get_area(self):
          """
          :returns: The area of the surface.
          
          .. warning:: The area is computed as the sum of the areas of all
              the polygons minus the sum of the areas of all the holes.
          """
          polys = sum([polygon.get_area() for polygon in self])
          holes = sum([hole.get_area() for hole in self.holes])
          return polys-holes