Rodrigo A. Botafogo Ehud Rivlin Ben Shneiderman Structural Analysis of Hypertexts: Identifying Hierarchies and Useful Metrics. 142-180 1992 10 ACM Trans. Inf. Syst. 2 http://doi.acm.org/10.1145/146802.146826 db/journals/tois/tois10.html#BotafogoRS92