1. W.J. Blok and D. Pigozzi. Algebraizable logic. Memoirs of the Amer. Math. Soc., 396, 1989.
2. C.C. Chang and H.J. Keisler. Model Theory. North-Holland, Amsterdam, 1973.
3. A. Ehrenfeucht. Decidability of the theory of linear ordering relation. Notices Amer. Math. Soc, 6:268–269, 1959.
4. Yu.L. Ershov, I.A. Lavrov, A.D. Taimanov, and M.A. Taislin. Elementary theories. Uspechi Matematicheskich Nauk, 20:37–108, 1965. In Russian.
5. R. Fagin, J.Y. Halpern, and M.Y. Vardi. What is an inference rule? Journal of Symbolic Logic, 57:1018–1045, 1992.