Statistical parsing on the Penn Treebank has made great progress by focusing on the machine-learning or algorithmic aspects (Magerman 1994;Ratnaparkhi 1998;Collins 1999;Charniak 2000;Henderson 2004;McDonald, Crammer, and Pereira 2005).
the null element *PPA* ("Permanent Predictable Ambiguity," 26 occurrences) is used for genuine attachment ambiguities.