Thierry Cachat Higher Order Pushdown Automata, the Caucal Hierarchy of Graphs and Parity Games. 556-569 2003 conf/icalp/2003 ICALP http://link.springer.de/link/service/series/0558/bibs/2719/27190556.htm db/conf/icalp/icalp2003.html#Cachat03