I forgot the URL http://www.jlcomp.demon.co.uk/13_unbalanced_indexes_i.html
http://www.jlcomp.demon.co.uk/faq/ind_faq.html The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/appearances.html Public Appearances - schedule updated 4th Nov 2005
There's an article on my website that mentions the idea, with a schematic of the state an index can get into.