1. Institute of Mathematics, Białystok University, ul. Akademicka 2, 15-267 Białystok, Poland, anna.gom@math.uwb.edu.pl
2. Dept. of Logic and Methodology of Science, The Maria Curie-Skłodowska University, pl. Maria Curie-Skłodowska 4, 20-031 Lublin, Poland, marcin.wolski@umcs.lublin.pl