\Bibitem{Zyk63} \by G.~P.~Zykin \paper Remark on a theorem of Hao Wang \jour Algebra i Logika. Sem. \yr 1963 \vol 2 \issue 1 \pages 33--35 \mathnet{http://mi.mathnet.ru/al977} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=0155748}