1. A. I. Mal'tsev, Algebraic Systems [in Russian], Nauka, Moscow (1970).
2. A. I. Mal'tsev, ?Model correspondences,? Izv. Akad. Nauk SSSR, Ser. Mat.,23, No. 3, 313?336 (1959).
3. R. Vaught, ?The elementary character of two notions from general algebra,? Essays on the Foundations of Mathematics, North-Holland, Amsterdam (1962), pp. 226?233.
4. M. Makkai, ?A compactness result concerning direct products of models,? Fund. Math.,57, No. 3, 313?325 (1965).
5. T. Kashiwagi, ?On certain compactness properties for classes of structures,? Math. Japonicae,21, 327?332 (1976).