Poberezhnyi, V. I., and O. V. Marchuk. “Issues of “pain” Phenomenon. Formulation of ‘pain’ Definition Based on System Approach”. Pain medicine 1, no. 1 (March 1, 2016). Accessed October 17, 2019. https://www.painmedicine.org.ua/index.php/pnmdcn/article/view/1.