B. Bollobas, and O. Riordan. (2008)cite arxiv:0812.2656
Comment: 44 pages, 1 figure. This is a companion paper to arXiv:0708.1919,
consisting of an updated version of part of the original version
(arXiv:0708.1919v1), which has been split into two papers. Only change from
v1 is to correct a reference.