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\newblock Addison-Wesley, second edition, 1973.

\bibitem{maclane67}
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\newblock {\em Algebra}.
\newblock Collier-MacMillan Limited, London, 1967.

\bibitem{mostow63}
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\newblock {\em Fundamental Structures of Algebra}.
\newblock McGraw-Hill Book Company, 1963.

\end{thebibliography}
