code: func (a Affine) Rotate(angle float64) Affine {
  	s, c := math.Sincos(angle)
  	return a.Mul(Affine{
  		+c, +s, +0,
  		-s, +c, +0,
  		+0, +0, +1,
  	})
  }
docs: // Rotate produces a clockwise rotation transform of angle, in radians.