© IGCS and ESGO 2019. Re-use permitted under CC BY-NC. No commercial re-use. Published by BMJ.  Open access 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, an indication of whether changes were made, and the use is non-commercial. See: http:// creativecommons.org/licenses/by-nc/4.0
Except where otherwise noted, this item's license is described as © IGCS and ESGO 2019. Re-use permitted under CC BY-NC. No commercial re-use. Published by BMJ. Open access 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, an indication of whether changes were made, and the use is non-commercial. See: http:// creativecommons.org/licenses/by-nc/4.0