Funding for this research was provided by:
Bulgarian National Science Fund (DN02/9 of 17.12.2016)
SRF Sofia University (80-10-207/26.04.2018)
Text and Data Mining valid from 2019-11-21
Received: 23 October 2018
Accepted: 20 September 2019
First Online: 21 November 2019
Consent for publication
: Not applicable.
: The authors declare that they have not competing interests.