http://www.lili.uni-osnabrueck.de/obema/henson.html
http://www.questia.com/googleScholar.qst;jsessionid=KchRxq3JVh0l1MvC4mTNQxdk1YcMhMhFvddpTdkLzFJT2TRJz1fb!-82436048!-1854981251?docId=95710687