http://pier.botik.ru/PSI/AIReC/logic/bst/bst.html http://www.acsu.buffalo.edu/~lizhang/Thesis/ http://www-diotima.math.upatras.gr/mirror/prncyb-l/0480.html http://www2.rz.hu-berlin.de/asg/blutner/afa/index.html (esp, lit and links (esp, Peter Aczel's hp) pages) http://www.dcs.ed.ac.uk/home/lego/ http://mizar.uw.bialystok.pl/JFM/