1. S.R. Alpert, Iterated current graphs, personal communication.
2. S.R. Alpert and J.L. Gross, Components of branched coverings of current graphs, in preparation.
3. Generators and Relators for Discrete Groups;Coxeter,1972
4. Branched coverings of current graph imbeddings;Gross;Bull. Am. Math. Soc.,1973
5. J.L. Gross and S.R. Alpert, The topological theory of current graphs, J. Combin. Theory, to appear.