1. The algebraic theory of semigroups;Clifford;American Mathematical Society,1961
2. The algebraic theory of semigroups;Clifford;American Mathematical Society,1967
3. A complete unification algorithm for associative-comunitative functions;Stickel;Proceedings of the 4th International Joint Conference on Artificial intelligence,1975
4. M. Livesey and J. Siekmann, Unification of A + C-terms (bags) and A + C + I-terms (sets), Interner Bericht Nr. 3/76, Institut für Iformatik I, Universität Karlstuhe.
5. An algorithm to generate the basis of solutios homogenoeus linear diophantine equations;Huet;Rapport Laboratoria no. 274,1978