M. Bridson, and K. Vogtmann. (2010)cite arxiv:1007.2598
Comment: Incorrect Lemma (2.5) replaced by new Proposition 2.5. Typos
corrected, references updated.
R. Trinchero. (2010)cite arxiv:1004.5104
Comment: To appear in the proceedings of "Colloquium on Hopf Algebras, Quantum
Groups and Tensor Categories", August 31st to September 4th 2009, La Falda,
Cordoba, Argentina..