Licence information
This article has been accepted for publication in BMJ, 2018 following
peer review, and the Version of Record can be accessed online at https://doi.org/10.1136/bmj.k1121.
© 2018. Reuse of this manuscript version is permitted
strictly pursuant to the terms of the CC-BY-NC-ND 4.0 license:
http://creativecommons.org/licenses/by-nc-nd/4.0/