@article{KR65,
  title={A general theorem on selectors},
  author={Kuratowski, Kazimierz and Ryll-Nardzewski, Czes{\l}aw},
  journal={Bull. Acad. Polon. Sci. S{\'e}r. Sci. Math. Astronom. Phys},
  volume={13},
  number={6},
  pages={397--403},
  year={1965}
}