M. Aulbach, M. Fink, J. Schuhmann, and A. Wolff. Proceedings of the 22nd International Symposium on Graph Drawing (GD '14), volume 8871 of Lecture Notes in Computer Science, page 367-379. Springer-Verlag, (2014)
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.