Shinkov, A
Borissova, A-M
Dakovska, L
Vlahov, J
Kassabova, L
Svinarov, D
Article History
Received: 10 February 2014
Revised: 2 July 2014
Accepted: 10 July 2014
First Online: 13 August 2014
Competing interests
: The authors declare no conflict of interest.