Kurteva, Ekaterina Krasimirova http://orcid.org/0000-0003-1968-5962
Boyadzhieva, Vladimira Vasileva http://orcid.org/0000-0002-6780-5810
Stoilov, Nikolay Rumenov http://orcid.org/0000-0001-7183-086X
Article History
Received: 16 July 2019
Accepted: 8 January 2020
First Online: 22 January 2020
Compliance with ethical standards
:
: The authors declare that they have no confict of interest. No part of the manuscript has been copied or published elsewhere.