. 2018 May 14;8(5):e017819. doi: 10.1136/bmjopen-2017-017819
© Article author(s) (or their employer(s) unless otherwise
stated in the text of the article) 2018. All rights reserved. No commercial use is
permitted unless otherwise expressly granted.
This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/