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For the methods of syntactic parsing and semantic parsing, it is pointed out that due to the distance between linguistic theories and actual application of natural languages,parsing should comprise both grammars and algorithms. (T.J. Zhao, 2000, p.156) And it is also pointed out that grammars are only the illustrative representation of knowledge structure of languages. (R.Q. Lu, 1996, p.955) When grammars function as the tool of the parsing of languages, grammars are referred to as language recognizers. Here the specific recognition processes are not included in the grammars overtly. Therefore, in order to construct the actual recognition processes, it is necessary to have another representation method, which is referred to as automata such as pushdown automata.(Lewis&Papadimitriou, 1998)
Head-driven phrase structure grammar (HPSG) is an integrated theory of natural language syntax and semantics. (Pollard&Sag, 1994)
In HPSG word, phrase and sentence are all signs. Signs have phonological, syntactic, semantic and discourse attributes. One way to represent attribute structure is
through attribute-value matrix (ABM). (Pollard&Sag, 1994)
HPSG consists of rule schemata and lexical entries. The attribute structure of a lexical entry is the total sum of its attributes and the values of these attributes. The attribute structure of a schema is the sum of attribute structures of the lexical entries in the schema. The attribute structure of a sentence is the sum of attribute structures of the schemata in the sentence. (Pollard&Sag, 1994)
The mother node in a schema is the head. The reason is as follows:
HPSG has two characteristics: 1 .It inherits the principles of GPSq and is a grammar based on constraints. 2. In the meantime, it assimilates the strong points of LFq http://www.ukthesis.org/yyxzy/ emphasizes the importance of lexicon in language construction. The main feature
of this grammar is that it emphasizes the role that heads play in parsing, and the whole grammatical system is head-driven. (Pollard&Sag, 1994)
HPSG embodies the process of imparting head information through the SUBCAT
feature of lexical items. In HPSq the SUBCAT feature is made of a constituent list,
which describes in detail the features of the lexical item which is a head. (Pollard&Sag,1994) Heads in phrase structure grammar refer to constituents that determine the main functions of a sentence or phrase when a sentence or phrase is constructed. For example,in a verb phrase, the verb is the head of this phrase. (T. J. Zhao, 2000)
Since HPSG particularly highlights the role which head plays, according to SUBCAT feature of the head, it is possible that the grammatical information of the head as well as other constituents can be associated very conveniently, such that information of the whole sentence is associated with the heads as the kernel. Complex attributes can be used to represent information of sentences, and this facilitates the processing. (T. J.Zhao, 2000)
