Ahmed Bouajjani Peter Habermehl Lukás Holík Tayssir Touili Tomás Vojnar Antichain-Based Universality and Inclusion Testing over Nondeterministic Finite Tree Automata. 57-67 2008 CIAA http://dx.doi.org/10.1007/978-3-540-70844-5_7 conf/wia/2008 db/conf/wia/ciaa2008.html#BouajjaniHHTV08