1. D.J. Brown andR. Suszko,Abstract logics,Dissertationes Mathematicae, Vol. 102 (1973) pp. 5–41.
2. G. Gratzer,Universal algebra, Princeton (1968).
3. S. L. Bloom andD. J. Brown,Classical abstract logics,Dissertationes Mathematicae, Vol. 102 (1973) pp. 43–52.
4. H. Rasiowa andR. Sikorski,The mathematics of metamathematics Warsaw (1963).