D-Tree Substitution Grammars Owen Rambow author K Vijay-Shanker author David Weir author 2001 text journal article Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal rambow-etal-2001-tree 10.1162/089120101300346813 https://aclanthology.org/J01-1004/ 2001 27 1 87 121