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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 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)
The attribute passing mechanism of HPSG is unification and structure-sharing.
(This is not required in general attribute grammar.) Unification refers to the existence of maximum unification. For two attribute structures Xl and X2, if another attribute structure X3 is subsumed in both X1 and X2, then X3 is called a unification of X1 and X2. And if for any unification X of X1 and X2, X is subsumed in X3, then X3 is the maximum unification of X1 and X2. (R.Q. Lu, 1996, p.969) Structure sharing means that the attribute value of the mother node of the lower schema is passed to the daughter node of the upper schema.   
The attribute passing mechanism of HPSG is embodied in schemata and principles.Immediate (ID) schemata. Immediate (ID) schemata in HPSG occupy a position in the theory analogous to that of X一schemata in GB theory: they are a small, universally available set of disjunctive constraints on the immediate constituency of phrases, from among which each language makes a selection an thus the disjunction of the ID schemata itself constitutes a universal principle, which we  call the Immediate Dominanoe Prinoiple (IDP). (Pollard&Sag, 1994)   
And for principles, in HPSq there are some very important principles, for example,the head feature principle (HFP), the subcategorization principle, the trace principle,and the semantic principle. We will introduce these principles which will be used in later chapters of this dissertation.

The semantic principle is defined as follows:
a. the RETREIVED value is a list whose set of elements forms a sub-set of the union of the QSTOREs of the daughters; and the QSTORE value is the relative complement of that set   
b. (Case 1) if the semantic head's CONTENT value is of sort psoa, then the NUCLEUS value is identical with that of the semantic head, an本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。