Funding for this research was provided by:
Ministry of Science, ICT and Future Planning (NRF-2016M3A9D5A01952416)
Text and Data Mining valid from 2018-08-27
Received: 23 April 2018
Accepted: 9 August 2018
First Online: 27 August 2018
Compliance with ethical standards
: All authors declare that they have no conflict of interest.