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2. Iteration of expansions, unambiguous semi-groups;Birget;J. Pure Appl. Algebra,1984
3. Almost finite expansions of arbitrary semigroups;Birget;J. Pure Appl. Algebra,1984
4. Combinatorial group theory;Lyndon;Ergeb. Math. Grenzgeb.,1977
5. Combinatorial Group Theory;Magnus,1976