code: def invert(self):
          
          for poly in self.polygons:
              poly.flip()
          self.plane.flip()
          if self.front: 
              self.front.invert()
          if self.back: 
              self.back.invert()
          temp = self.front
          self.front = self.back
          self.back = temp
docs: Convert solid space to empty space and empty space to solid space.