1. I. Barwice and A. Robinson, ?Completing theories by forcing,? Ann. Math. Logic, No. 2, 119?142 (1970).
2. A. Robinson, ?Forcing in model theory,? in: Symposia Mathematica, Vol. 5 (INDAM, Rome, 1969/70), Academic Press, London (1971), pp. 69?82.
3. Yu. L. Ershov, E. A. Palyutin, and M. A. Taitslin, Mathematical Logic [in Russian], Novosibirsk State Univ. Press, Novosibirsk (1973).
4. B. H. Neumann, ?Algebraically closed semigroups,? in: Studies in Pure Math., Academic Press, New York (1970), pp. 185?194.
5. L. A. Bokut', ?Embedding theorems for rings and semigroups. I,? Sib. Mat. Zh.,4, No. 3, 500?517 (1963)