All articles published in the Journal are copyrighted to the authors (©The Authors).

All articles in the Journal are open access, licensed under a Creative Commons Attribution 4.0 International License (CC BY), which means that anyone may download, distribute, and adapt for commercial or non-commercial purposes for free, while citing Tsinghua University Press as the original publisher and ensuring that the authors receive proper credit. The CC-BY Creative Commons attribution license does not apply to third-party materials that display a copyright notice to prohibit copying. Unless the third-party content is also subject to a CC-BY Creative Commons attribution license, or an equally permissive license, the author(s) must comply with any third-party copyright notices. To get more information of this licence, please visit http://creativecommons.org/licenses/by/4.0/.