def rotate(mat)
      rot_start_point = mat*start_point
      rot_end_point = mat*end_point
      return FiniteLine.new(rot_start_point, rot_end_point)
    end
def rotate(mat)
      rot_start_point = mat*start_point
      rot_end_point = mat*end_point
      return FiniteLine.new(rot_start_point, rot_end_point)
    end
[Input]
  _mat_ should be Matrix which row and col size are 3.
 [Output]
  return rotated FiniteLine.