code: def rotate(self, angle, center=(0, 0)):
          
          ca = numpy.cos(angle)
          sa = numpy.sin(angle)
          sa = numpy.array((-sa, sa))
          c0 = numpy.array(center)
          self.polygons = [(points - c0) * ca + (points - c0)[:, ::-1] * sa + c0
                           for points in self.polygons]
          return self
docs: Rotate this object.
  
          Parameters
          ----------
          angle : number
              The angle of rotation (in *radians*).
          center : array-like[2]
              Center point for the rotation.
  
          Returns
          -------
          out : ``PolygonSet``
              This object.