HEAD-DRIVEN PHRASE STRUCTURE GRAMMAR
One of the direct followers of the GPSG was called Head-Driven Phrase Structure Grammar (HPSG). In addition to the advanced traits of the GPSG, it has introduced and intensively used the notion of head. In most of the constituents, one of the sub-constituents (called daughters in HPSG) is considered as the principal, or its head (called also head daughter). For example, in the rule:
S ® NP(Pers, Num) HVP(Pers, Num),
the VP constituent (i.e., the syntactic predicate of a sentence with all connected words) is marked as the head of the whole sentence, which is indicated by the symbol H. Another example: in the rule:
NP (Gen, Num) ® D (Gen, Num) HN (Gen, Num),
the noun is marked as the head of the whole noun phrase NP.
According to one of the special principles introduced in HPSG, namely the head principle, the main features of the head are inherited in some way by the mother (enclosing) constituent (the left-side part of the rule).
In the previous examples, the features of the predicate determine features of the whole sentence, and the features of the noun determine the corresponding features of the whole noun phrase. Such formalism permits to easier specify the syntactic structure of sentences and thus facilitates syntactic analysis (parsing).
As it was already said, the interpretation in early generative grammars was always of syntactic nature. For semantic interpretation (“understanding”), additional theoretical means were introduced, which were somewhat alien to the earlier generative structure mainstream. By contrast, each word in the HPSG dictionary is supplied with semantic information that permits to combine meanings of separate words into a joint coherent semantic structure. The novel rules of the word combining gave a more adequate method of construing the semantic networks. Meantime, Chomskian idea of transformations was definitely abandoned by this approach.
THE IDEA OF UNIFICATION
Having in essence the same initial idea of phrase structures and their context-free combining, the HPSG and several other new approaches within Chomskian mainstream select the general and very powerful mathematical conception of unification. The purpose of unification is to make easier the syntactic analysis of natural languages.
The unification algorithms are not linguistic proper. Rather they detect similarities between parts of mathematical structures (strings, trees, graphs, logical formulas) labeled with feature sets. A priori, it is known that some features are interrelated, i.e., they can be equal, or one of them covers the other. Thus, some feature combinations are considered compatible while met in analysis, whereas the rest are not. Two sets of features can be unified, if they are compatible. Then the information at an object admitting unification (i.e., at a constituent within a sentence to be parsed) combines the information brought by both sets of features.
Unification allows filtering out inappropriate feature options, while the unified feature combination characterizes the syntactic structure under analysis more precisely, leading to the true interpretation of the sentence.
As the first example of unification operations, let us compare feature sets of two Spanish words, el and muchacho, staying in a text side by side. Both words have the feature set [gender = masculine, number = singular], so that they are equivalent with respect to gender and number. Hence, the condition of unification is satisfied, and this pair of words can form a unifying constituent in syntactic analysis.
Another example is the adjacent Spanish words las estudiantes. The article las has the feature set [gender = feminine, number = plural]. As to the string estudiantes, this word can refer to both ‘he-student’ of masculine gender and ‘she-student’ of feminine gender, so that this word is not specified (is underspecified) with respect to gender. Thus, the word occurrence estudiantes taken separately has a broader feature set, namely, [number = plural], without any explicit indication of gender. Since these two feature sets are not contradictory, they are compatible and their unification [gender = feminine, number = plural] gives the unifying constraint set assigned to both words. Hence, this pair can form a unifying mother constituent las estudiantes, which inherits the feature set from the head daughter estudiantes. The gender of the particular word occurrence estudiantes is feminine, i.e., ‘she-students,’ and consequently the inherited gender of the noun phrase las estudiantes is also feminine.
As the third example, let us consider the words niño and quisiera in the Spanish sentence El niño quisiera pasar de año. The noun niño is labeled with the 3rd person value: [person = 3], whereas the verb quisiera exists in two variants labeled with the feature set [person = 1 or person = 3], correspondingly. Only the latter variant of the verb can be unified with the word niño. Therefore this particular word occurrence of quisiera is of the third person. The whole sentence inherits this value, since the verb is its head daughter.