Trace Prediction and Recovery with Unlexicalized PCFGs and Slash Features

This paper describes a parser which generates parse trees with empty elements in which traces and fillers are co-indexed. The parser is an unlexicalized PCFG parser which is guaranteed to return the most probable parse. The grammar is extracted from a version of the PENN treebank which was automatically annotated with features in the style of Klein and Manning (2003). The annotation includes GPSG-style slash features which link traces and fillers, and other features which improve the general parsing accuracy. In an evaluation on the PENN treebank (Marcus et al., 1993), the parser outperformed other unlexicalized PCFG parsers in terms of labeled bracketing fscore. Its results for the empty category prediction task and the trace-filler coindexation task exceed all previously reported results with 84.1% and 77.4% fscore, respectively.