Text and Data Mining valid from 2019-04-01
Received: 3 October 2018
Accepted: 14 February 2019
First Online: 1 April 2019
Compliance with ethical guidelines
: D. Martin, C. Rödel and E. Fokas declare that they have no competing interests.
: All human studies have been approved by the appropriate ethics committee and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.