N. P. Matiytsiv Ivan Franko National University of Lviv Ukraine
K. A. Dronska Ivan Franko National University of Lviv Ukraine
N. M. Artymovych Ivan Franko National University of Lviv Ukraine
A. M. Makarenko Taras Shevchenko National University of Kyiv Ukraine
Ya. I. Chernik Ivan Franko National University of Lviv Ukraine