\Bibitem{Gre72} \by P.~G.~Gres' \paper Character algebras \jour Algebra Logika \yr 1972 \vol 11 \issue 6 \pages 633--647 \mathnet{http://mi.mathnet.ru/al1356} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=0318277}