On ACM Portal
http://portal.acm.org/citation.cfm?id=1240866.1241101&coll=Portal&dl=ACM&CFID=72648415&CFTOKEN=79180016
Local
file:///Users/nori/Documents/RESEARCH/REFERENCES/2007/p2881-willis.pdf
http://www.springerlink.com/content/u7h9v5/?p=f45057f836204675bc6d99b842415479&pi=0