: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.

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