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Article History

Received: 27 October 2019

Accepted: 27 April 2020

First Online: 26 May 2020

Compliance with ethical guidelines

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: L. Zisova, V. Valchev, and G. Kasabov declare that they have no competing interests.

: The authors confirm that the ethical policies of the journal, as noted on the journal’s author guidelines page, have been adhered to. All procedures performed in studies involving human participants or on human tissue were in accordance with the ethical standards of the institutional and/or national research committee and with the 1975 Helsinki declaration and its later amendments or comparable ethical standards. This study obtained ethical approval from the Scientific Ethics Committee of Medical University Plovdiv with chairmen Prof. Dr. Mariyana Stoycheva. Informed consent was obtained from all individual participants included in the study.