Robust syntactic analysis requires a very broad coverage grammar and means for dealing with sentences that do not parse, whether because they fall outside the coverage of the grammar or because they are too long for the parser. The grammar used in TACITUS is that of the DIALOGIC system, developed in 1980-81 essentially by constructing the union of the Linguistic String Project Grammar (Sager, 1981) and the DIAGRAM grammar (Robinson, 1982) which grew out of SRI's Speech Understanding System research in the 1970s. Since that time it has been considerably enhanced. It consists of about 160 phrase structure rules. Associated with each rule is a ``constructor'' expressing the constraints on the applicability of that rule, and a ``translator'' for producing the logical form.
The grammar is comprehensive and includes subcategorization, sentential complements, adverbials, relative clauses, complex determiners, the most common varieties of conjunction and comparison, selectional constraints, some coreference resolution, and the most common sentence fragments. The parses are ordered according to heuristics encoded in the grammar.
The parse tree is translated into a logical representation of the meaning of the sentence, encoding predicate-argument relations and grammatical subordination relations. In addition, it regularizes to some extent the role assignments in the predicate-argument structure, and handles arguments inherited from control verbs.
Our lexicon contains about 20,000 entries, including about 2000 personal names and about 2000 location, organization, or other names. This number does not include morphological variants, which are handled in a separate morphological analyzer.
The syntactic analysis component was remarkably successful in the MUC-3 evaluation. This was due primarily to three innovations.