\Bibitem{Oma91} \by A.~I.~Omarov \paper Filtering formulas \jour Algebra Logika \yr 1991 \vol 30 \issue 6 \pages 693--704 \mathnet{http://mi.mathnet.ru/al2174} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=1213730}