Teodor Knapik Damian Niwinski Pawel Urzyczyn Higher-Order Pushdown Trees Are Easy. 205-222 2002 conf/fossacs/2002 FoSSaCS http://link.springer.de/link/service/series/0558/bibs/2303/23030205.htm db/conf/fossacs/fossacs2002.html#KnapikNU02