I \\ - I Complements and Adjuncts in Dependency Grammar~arsing Emulated by a Constrained Context-Free Gramn~ar Tom B.Y. Lai Changuing Huang Dept. of Chinese, Translation and Dept. of Computer Science and Technology Linguistics Tsinghua University, Beijing City University ofHong Kong Beijing 100084 Tat Chee Avenue, Kowloon China Hong Kong [email protected] Dept. of Computer Science and Technology Tsinghua University, Beijing [email protected] (b) All others depend directly on some element; (c) No element depends directly on more than one other;, Abstract (d) If A depends directly on B and some element C intervenes between them (in linear order ofs~ing), Generalizing from efforts parsing natural then C depends directly on A or on B or some other language sentences using the grammar intervening element. formalism, Dependency Grammar (DG) has been emulated by a context-free grammar (CFG) These axioms require that all words should constrained by grammatical function annotation. depend on only one word and that, arranging Single-headedness and projectivity are assumed. words in linear order, crossing of dependency This approach has the benefit of making general links as in Fig.l should not be allowed. constraint-based context-free grammar parsing facilities available to DG analysis. This paper describes an experimental implementation of this approach using unification to realize grammatical function constraints imposed on a dependency structure backbone emulated by a context-flee grammar. Treating complements of a head word using subcategoHzation lists Yuyanxue wo zhidao ta xihuan residing in the head word makes it possible to linguistics I know he likes keep phrase-structure-rule-like mechanisms to the minimum. Adjuncts are treated with a syntactic mechanism that does not reside in the Fig.! lexicon in this generally lexical approach to grammar. These are effectively the requirements of single- headedness and projectivity. Introduction While there are some schools of DG that do not follow Robinson's axioms in their entirety (e.g. The mathematical properties of Dependency Hudson (1984, 1990), Melcuk (1988)), many Grammar (Tesniere (1959)) are studied by computational linguists working on DG-based Gaifman (1965) and Hays (1964). Following their parsing have based their work on these footsteps, Robinson (1970) formulates four assumptions (e.g. Hellwig (1986), Covington axioms to govern the weU-formedness of (1990)). DG parsing of Chinese have used dependency structures: statistical corpus-based algorithms (Huang et al. (1992), Yuan and Huang. (1992)), rule-based (a) One and only one element is independent; 102 they may or may not take word order into defined by Robinson's axioms, does have aspects consideration; but they all observe Robinson's that cannot be modelled elegantly by PSG. axioms in their entirety. They also label I depedency relations with grammatical functions 1 Representation of Dependency like subject and object. Generalizing over DG-based parsing of The governor-dependent (head-modifier) I Chinese, Lai and Huang (1994) note that, taking relationship between words in an utterance can be linear word-order into consideration, this represented as for the Chinese sentence (from approach to DG can be emulated by a model YuanandHuang(1992) in Fig. 2: I having a context-free constituent component that / '~ is constrained by a grammatical function / component, very much in the spirit of the constituent and functional structures of Lexical Functional Grammar (LFG, Bresnan ed. (1982)). U The syntactic dependency structure in this Na ten zai gongyuan li approach to DG is however different from that person in park inside context-free phrase structure in that non-lexical phrasal nodes are not allowed. As in LFG, the Fig. 2 grammatical function structure, which provides the constraining mechanism, is mathematically a The main (or central) element, using Hays' (1964) graph rather than a tree. This relieves the syntactic terminology, of the sentence is zaL Its immediate dependency component of any need to be dependants are ten and !i, which, in turn, have multiple-headed and non-projective (Lai and dependants of their own. This sentence can also be Huang 1995). Following this approach, Lai and represented as in Fig. 3 (Tesniere's stemma): Huang (in press) describes a unification-based (Shieber (1986)) experimental parser adapted zai from the PATR parser in Gazdar and Mellish (1989). Control and simple semantic analysis are ten ii handled. / na gongyuan The present paper discusses issues of using a constrained CFG to emulate DG. Section 1 Fig. 3 explains the implications of Robinson's axioms and describes Hays' CFG-like formulation of If we do not mangle up word order in the dependency rules. Section 2 formulates the dependency structure of Fig. 3, it can be seen that "dependency rule with grammatical function it is equivalent to the tree structure in Fig. 2. annotation" model and describes its emulation Based on the work of Gaifman (1965), Hays with PATR. Section 3 discusses how the lexical (1964) proposes rules of the following form for orientation of DG motivates a proper distinction the generation of dependency structures: between complements subeategorized for by the head and adjuncts that are not, and describes how (a) X(A,B,C..... H, * ,Y.... ,Z) this can be accomplished in a constrained CFG (I,) X(* ) emulation using subcategorization lists in the (c) * (X) lexicon. A distinction between grammatical information residing and not residing in the Fig. 4 lexicon is noted. Section 4 discusses the real nature of the constrained CFG emulation of DG. In Fig.4, (a) states that the governing auxiliary Though DG can be usefully emulated by a alphabet X has dependent A, B, C ..... H, Y ..... Z constrained CFG model, the formalism, as least as and that X itself (the governor) is situated between 103 I I H and Y. Co) says that the terminal alphabet X LFG, they proposed annotated dependency rules occurs without any dependants. (c) says that X of the following form: occurs without any governor, i.e. it is the main or I central element. Gaifman (1965) establishes that a X(A(fa), B(~),...,* .... Z(fz)) Dependency Grammar obtained in this way is equivalent to a phrase structure grammar in the For example, the following rules account for the I sense that: transitive verbs: - they have the same terminal alphabet; (a) * (TV) - for every string over that alphabet, Co) TVfS(subj),* , N(obj)) I every structure attributed by either (c) N(* ) grammar corresponds to a structure attributed by the other. Fig. 6 I Robinson's (1970) four axioms (v. supra) While this is not a phrase-structure grammar license the same kinds of dependency structures (PSG), Lai and Huang (in press) exploited the I as Hays' rules. Unlike these rules, they do not obvious affinity between Robinson-style DG and contain unnecessary stipulation involving linear PSG and implemented a DG parser using the word order. It is easy to see that the third axiom is PATR of Gazdar and Mellish (1989). In this I a requirement of single-headedness. As for the implementation, the Chinese sentence fourth axiom, consider Fig. 5: B' Zhang San kanjian Li Si name saw name .is accounted for by the annotated rules (simplified and semantic analysis mechanisms stripped for ! brevity) in Fig. 7: Fig. 5 R ule TVP m> [PNI, TV, PN2] :- TVP:cat J" tv, The axiom stipulates that the governor of C must TV:cat ~ tv, be located between A and B (which are PNi:cat ~ n, themselves possible governors). Seen in the PN2:cat ~ n, Baysian cast, this effectively requires that the link TVP:ds ..... TV:ds, TVP:ds:subj --~ PNl:ds, between C and its governor should not cross the TVP:ds:obj " PN2:ds. lines AB' and BB'. This is thus a requirement of projectivity. W ord kanjian:- W:cat tv, 2 Dependency Rules and Grammatical W:ds:head .. kanjian. Function Annotation Word 'zhangsan':- W:cat .... n, Generalizing over approaches adopted in DG- W:ds:head ~ 'zhangsan'. based parsing of Chinese, Lai and Huang (1994) W ord 'lisi' :° noted that grammatical functions like subject and W:cat n, object are generally used to label dependency W:ds:head ~ ']isi'. links. These labels are not found in Hays' dependency rules and Robinson's axioms. In a Fig. 7 sense, they are entities on a second level. Borrowing the idea of functional annotation from 104 ! I Besides outputting a grammatical function V:ds:obj:filler' : N:ds. structure, the following dependency structure is Word xiang3 :- produced: W:cat : v, i W:subcat:tran ~ tv, [tv, [[n, [zhangsan]], [tv, [kanjian]], [n, [lisi]]]] W:subcat:c~'l II l subj, W:ds:head ~.: xiang. i PATR outputs a phrase-structure tree, but after Control sentences are one of the motivations applying a pruning operation on the two tv's, a for DG grammarians like Hudson (1994) to give structure equivalent to Fig. 8 is obtained: up the single-headedness and non-projectivity I requirements, finding it difficult, for example, not TV kanjian to allow the controlled verb da to have both Zhang San and xiang as its heads, violating single- I ~ headedness and projectivity at the same time as / \ shown in Fig. 9: I Zhang San Li Si Fig. $ I The more complicated sentence, involving a subject-control verb, Zhang San xiang da Li Si Zhang San xiang da Li Si Fig. 9 name want hit name By introducing a level of grammatical function to is accounted for the following rules (necessary accommodate such complications, Lai and Huang details only, for brevity): (1995; in press) preserve single-headedness and projectivity in the syntactic dependency structure Rule CVP ---> [N, CV, VS] :- CVP:slash:here =--~ no, as in Fig. 10: CVP:slash:down ~ yes, CVP:cat --- CV:cat, CV:cat ~ v, CV:subcat:tran ~ tv, CV:subcat:ctrl =~ subj, N:cat ...