TAG differs from CCG in having a smaller set of combinatory operations, but a more varied set of elementary objects: the TAG-annotated version of the Penn Treebank that we use (Chen, 2001) includes 4727 distinct supertags (2165 occur once) while the CCG-annotated version (Hockenmaier and Steedman, 2007) includes 1286 distinct supertags (439 occur once).
building a robust, broad-coverage TAG parser has proven difficult.