Recovering a Logical Form Representation Using

Recovering a Logical Form Representation Using a Single-Pass Principle-Based Parser1 Carl Alphonce ([email protected]) Department of Computer Science University of British Columbia Vancouver, BC V6T 1Z2

Abstract

This is in contrast to language-specific parsers, such as those based on context-free rule sets. Such parsers cannot easily handle more than one language, and their degree A problem in using principle-based parsers in building natof coverage is dependent on the particulars of the rule set ural language software tools centres around the difficulty which they employ. of recovering information represented at various levels of the grammar. The information encoded in the representa- Although principle-based parsers are in theory very tions at the levels of D-Structure, S-Structure, and Logical promising, they are not always easy to construct. While Form (LF) increases monotonically. While D-Structure there are a number of well-known and efficient parsing and S-Structure are relatively easy to recover, LF presents techniques which can be used with context-free rule sets, numerous problems for a single-pass parser. Some of of there does not seem to be any simple and straightforward the constraints which apply to the human parsing mech- way to parse all languages using a single-pass principleanism are considered, and a single-pass principle-based based parser.2 Mazuka and Lust [25] claim that it is not parser which recovers an LF representation is presented. possible to employ a single parsing scheme for all lanThis single parsing mechanism is shown to parse both En- guages, suggesting instead that the parser must be paglish and Chinese wh-questions. The mechanism which rameterized according to the branching direction of the allows it to deal with English and Chinese also solves a language. I reject this approach for two reasons. First, parsing dilemma involving some complex English con- languages are in general not purely left-branching or rightstructions. branching, but rather have a mixture of branching patterns for different X projections. Thus, it does not seem possible to parameterize the parser in this way to begin with. Second, I believe that allowing the parser to be parameterized 1 Introduction results in too powerful a mechanism. Not allowing the parser to be parameterized results in a more constrained In recent years, principle-based parsers have become ever theory, one which can yield stronger, more testable premore popular; see, among others, [2, 4, 6, 10, 11, 13, 21, dictions, than one in which both the syntactic theory and 24, 28]. The linguistic theory underlying these parsers, the parser is parameterized. Government-Binding theory in one form or another, is a This paper presents a principle-based parsing scheme modular theory which strives to achieve a high degree of which can recover an LF representation for a given inempirical coverage and cross-linguistic applicability in a put in a single-pass. The methodological approach taken psycholinguistically explanatory fashion. The attraction is exploit the constraints under which the human language of a principle-based parser is that with a proper architecprocessing system must operate to arrive at a psycholinture, such a parser will have a broad coverage, will be guistically plausible computational model. highly modular, and will be easily adaptable to a variety of languages with little (if any) modification to the code. 1 I would like to thank Henry Davis for many helpful discussions, Yan Feng Qu for insight into Chinese, and three anonymous PACLING reviewers for useful comments. I also thank NSERC for financial support in the form of a Post Graduate Scholarship. Any errors remain, of course, my own.

2 Fong [13] showed in his thesis that it is quite possible to construct a principle-based parser which can handle a wide range of data in a number of languages. However, Fong’s parser constructs the various levels of representation one at a time.

Lexicon

2 Parsing Assumptions In order to limit the form of the solution as much as possible, several assumptions about the parsing mechanism are made. I consider the human parser to be the prime example of a universal language processing mechanism, and therefore impose psycholinguistically motivated constraints on the parsing scheme. These constraints, which originate in one or more of [14, 15, 19, 24], are outlined presently.

   

D-Structure

S-Structure

Phonological Form (PF)

Logical Form (LF)

Figure 1: The different levels of the grammar The parser proceeds from left to right, processing the input in the order in which people would hear it if it were spoken.

3.1 A Multistratal Theory

The parser operates in a primarily bottom-up manner, GB theory is a multistratal theory, meaning that it postuprojecting structure from the level of the individual lates the existence of several different levels of syntactic lexical items. representation. The different levels of the grammar are shown in figure 1. D-Structure is the level at which theThe parser is filler-driven, meaning that only once a matic relations are transparently expressed; Logical Form dislocated element has been identified will the parser (LF) is the level at which operator scope is expressed, begin to search for the extraction site of the element. and serves as an interface to the semantics; Phonological Finally, the parser is deterministic in Marcus’ sense. Form (PF) is meant to deal with post-lexical phonology, and is the grammar’s interface with the motor-perceptual This entails, among other things, that the parser, system; finally, S-Structure can be viewed as an (internal) interface between LF and PF. This paper deals proposes a – cannot encode structure in its internal state, – cannot discard any structure which it builds dur- parsing model which can recover an LF representation in a single pass. ing a parse, – may make use of only a limited lookahead facility, and – may not backtrack, save in the case of “gardenpath” input.

3 Syntactic Framework If one’s goal is to develop natural language software tools which can easily be adapted to different linguistic environments, it is obviously important that the underlying syntactic theory is cross-linguistically applicable. GovernmentBinding (GB) theory was developed with the goal of being a universal grammar. Towards that end, GB theory is a modular theory. A number of parameters is associated with each module; different settings of these parameters are assumed to account for cross-linguistic variation. This section is a very brief outline of GB theory. Because of space constraints, little motivation or justification is provided; only those aspects of the theory which are essential to understanding the remainder of the paper are discussed. A more complete presentation can be found in general GB references such as [9, 16, 20, 31].

3.2 X theory X theory is a highly generalized theory of the structure of phrases. The phrasal categories are generally divided into the lexical categories (such as noun, verb, adjective, and preposition) and the non-lexical or functional categories (such as inflection, complementizer, and determiner). Each category can head a projection, according to the Xschema [8]:3 X ?! Y* X X ?! X Z*

, ,

Y = specifier Z = complement

X theory does not directly encode any constraints on the linear order of elements.4 Instead, it primarily imposes a 3X

is known as the head of the projection; X is the single bar-level projection (of X); X, the maximal projection (of X), is sometimes written as XP. 4 By this I mean that there is no ordering of elements imposed on the right hand side of each individual rule in the schema. This schema does encode linear constraints indirectly because crossing branches are not allowed. For example, the specifier could never occur between the head

hierarchical organization. The linear order of elements in a particular rule is determined by parameter settings — one parameter specifies whether heads are initial or final, while another specifies the same for specifiers.

as well. In English, wh-phrases represent their scope at SStructure (and LF), while quantifiers do so exclusively at LF. The situation is reversed in Chinese, where quantifiers make explicit their scope at S-Structure (and LF), while wh-phrases show their scope only at LF [17, 23].

3.3 Move- and Chains

The scope of an operator-type element is the subtree which it c-commands at LF. It is assumed that wh-phrases must occupy the specifier of the CP over which they have scope. In English, wh-phrases move into this position by S-Structure; we thus have,

Syntactic representations at different levels of the grammar must obviously be related to one another in some manner. This relationship is captured in the rule “move- ”. Put crudely, move- allows the movement of any subtree of (4) Whoi does Ian believe [CP that Ann likes ti ] a tree to a new position on the tree, subject to various In Chinese, however, this movement takes place after Sconstraints. For example, the sentence, Structure, by LF. (1) John was tricked. (5) Ian xiangxin Ann xihuan shui? has the D-Structure representation shown in (2). Ian believe Ann like ASP who “Who does Ian believe that Ann likes?” (2) [ e was tricked John ] In this representation, e denotes a position which is generated empty at D-Structure. This is the position into which the phrase “John” will move at S-Structure, yielding (3). (3) [ Johni was tricked ti ]

3.5 Extended Projection Principle The Projection Principle asserts that thematic relations established at D-Structure must hold at S-Structure and at LF.6 Chains of coindexed elements allow the thematic relationship between two elements to be maintained, even under movement.

Here ti denotes a trace, a phonetically empty category which has the same (non-phonetic) features as the moved phrase.5 The trace and the phrase which it shares features Natural language seems to require the presence of a subject with are coindexed; this coindexation serves to indicate in each clause. A string such as, which trace belongs with which moved phrase, as there (6) * Seems John is happy. may be multiple dependencies in a structure, all of which must be distinguished. is ill-formed (hence the “*”) because there is no subject A chain consists of some phrase and a possibly null se- in the main clause. This string is not in violation of the quence of traces associated with this phrase. Thus, the Projection Principle, as the verb “seem” does not assign chain C = (Johni ; t i) is formed by the application of a -role to its subject. The Extended Projection Principle move- to the representation in (2) to form the represen- (EPP) captures this requirement simply by stipulating that each clause must have a subject. tation in (3).

3.4 A-movement

3.6 pro and PRO

A-movement is a particular kind of movement involving GB theory holds that there are four types of empty operator type elements, and is motivated by considerations categories, defined by the features [anaphoric] and of scope. Operators include such elements as quantifiers [pronominal]. The four possibilities are, (some, all, every, and so on) and wh-phrases (who, what, which X, etc). +pronominal ?pronominal Operators must represent their scope in structural terms at LF, though they may do so at other levels of representation and its complement. 5 This is not an entirely accurate characterization, but is sufficient for present purposes. The binding features of the trace must also be considered (see section 3.6).

+anaphoric

?anaphoric

PRO pro

NP-trace wh-trace

6 In recent work (such as [9]), the Projection Principle is seen as derivative from the Principle of Full Interpretation, and thus not an independent, free standing principle in its own right. The actual status of this principle has no bearing on the work presented here.

PRO is a somewhat anomalous element, in that it must not be governed, and hence cannot receive Case marking.7 Browning [7] argues that empty operators are pro. Since they are empty categories, they must fall into the above classification system; within it, only PRO and pro are likely candidates. Since empty operators can occur in governed positions, but are not referential in the way that PRO is, Browning considers PRO a poor choice. Furthermore, she notes that null operators “may be governed, must be an argument, may head an A-chain, and must be identified via agreement” [p. 126]. Since these are all properties of pro, Browning suggests that this is a better choice.8

3.7 Predication

DP

predication

CP

feature percolation

C’

PRO SPEC-HEAD agreement C

IP

Figure 2: Relative clauses

XP

XP

Front of Buffer

XP

Rear of Buffer

Figure 3: The buffer The relationship between a predicate and its antecedent is called predication. Williams [32] defines both simple and complex predicates. Only complex predicates, which are of the form shown below, are of interest here. (7) a. [IP PRO [VP : : : ]] b. [CP PRO/wh [IP : : : ]] The relevant example is that of a relative clause, such as this object relative, (8) Ann brought the girl who Ian wanted to meet.

parser has two major data structures, a buffer (a first-in first-out data structure — see figure 3) as well as a main stack (a last-in first-out data structure — see figure 4). The Marcus parser allows a five item window of lookahead; this lookahead takes the form not of lexical items from the input stream, but rather partially 9 processed lexical items (they have undergone morphological analysis) as well as larger tree fragments. 10 The parser also uses some auxiliary data structures to store the current link of chain as it is being constructed.

which is assumed to that the structure,

On the basis of studies of the pause structure of sentences, Abney [2] suggests that a chunking parser is psycholin(9) [DP the girl ]i [whoi Ian wanted to meet ti ]i guistically more plausible than one which deals directly As can be seen in figure 2, following Browning [7], there with the lexical items. Abney’s chunking parser consists are several relationships which come into play. The pred- of a chunker and an attacher. The chunker processes one ication relationship holds between the antecedent and the or more lexical items, building a phrase structure fragment predicate. The CP predicate agrees with its head C through over those which are related through functional selection, feature percolation. Finally, C and the specifier of CP according to a chunking algorithm. These phrase structure fragments are then joined by the attacher to build a comagree by SPEC-HEAD agreement. plete parse tree. I adopt this two-part architecture, though at present the parser uses a fairly simplistic chunking algorithm — the chunker simply projects each lexical item 4 A GB-based Parsing Model to its full X projection, enforcing functional selection as early as possible in the case of determiners and inflection. This section describes the parsing model that is used as Consider as an example the case of a determiner, which a starting point for the discussion. For details about the functionally selects a noun phrase. Once a determiner has final parsing model adopted, as well as its implementation, been identified, the chunker will be actively looking for a see the appendix. Following the Marcus parser [24], the noun (to head the functionally selected NP) in the input 7 Again, in recent work [9], in contrast with earlier assumptions [8], it has been suggested that PRO must be Case marked, and thus governed, bringing PRO under the auspices of the Case filter. The consequences of this change in theory for my proposal will be noted throughout. 8 If PRO can be governed and Case marked then it is not clear that PRO would not be a possible choice as empty operator. The particular choice of category for empty operators has no effect on the proposal.

9 Clearly the parser will be dealing with partially processed input — it is not dealing with the sound stream that we hear, but rather with abstract structures which are the result of phonetic, morphological, and perhaps other analyses as well. 10 The Marcus parser can place whole constituents in the buffer if their grammatical function in the sentence is unknown. Thus, the buffer is used as workspace for the parser.

XP Top of Stack

XP

XP Bottom of Stack

is known as functional selection. For instance, the complement of the I (Inflection) node is always a VP, and the complement of the D (Determiner) node is always an NP. Thus, when one of these elements is identified, whatever category they functionally select is licensed (and required) to appear. scope assignment Operators are licensed to appear in a syntactic structure by virtue of their having to make explicit in structural terms their scope.

prohibition against vacuous quantification13 Natural language does not allow vacuous quantification, and Figure 4: The stack hence the presence of an operator licenses (and again, requires) the presence of a variable for the operator stream. When the noun is found, its skeletal X structure to bind. is projected and attached into the DP, thus satisfying the functional selection requirement. The parser thus uses a Extended Projection Principle (EPP) The EPP licenses (partially) bottom up strategy in that it does not project (and requires) the presence of a subject in each clause. structure until it has evidence for it in the input stream, and a (partially) top-down strategy in that it will actively seek certain items in the input in the case of functional selection. 5 LF and Parsing The skeletal X projections created by the chunker are placed at the rear the buffer. The attacher removes chunks from the front of the buffer, joining them to form the final parse tree. The chunker and attacher operate in tandem – the chunker supplies the attacher with enough chunks to satisfy its lookahead, which is fixed at two chunks.11

Given the relatively uncontroversial assumptions about the parser and the syntax, it might be expected that no difficulties should arise from the conjunction of these assumptions. Unfortunately, this is not the case — the prediction resulting from this combined set of assumptions is that languages such as Chinese, which do not have overt Abney [1] and Abney and Cole [3] suggest that licensing wh-movement, should be unparsable. Clearly, this is not conditions be used to build up phrase structure. The idea a desirable result. is that each element in a phrase structure representation must have a function12 — its appearance must somehow In order to see the problem, consider first the case of whbe licensed. The attacher uses various licensing conditions movement in English, to govern the attachment of chunks to each other. A variety of licensing conditions have been proposed [1, 3, 21]; the (10) Who did you kiss? ones I use are, The structure which the parser is to recover for this sentence is, The -criterion not only licenses the appearance of elements needing -roles, it requires (11) [CP whoi [IP did you [VP kiss ti]]] their appearance, since the -roles cannot be left unasWhen the parser encounters the wh-phrase “who”, it recsigned. ognizes that this is a dislocated element, and, following Case-marking The requirement of Case marking licenses its filler-driven strategy, begins to construct an A-chain. the appearance of elements needing Case. The parser creates a coindexed trace of the wh-phrase, and places it in a special movement store while the chain is functional selection The complements of certain syntacbeing constructed. Once the endpoint of the chain has tic categories are always of the same category; this been located in the verb phrase, the trace is deposited in 11 Note that this gives a more reasonable account of lookahead than this position, and the parse is complete.14

-role assignment

simply saying that X lexical items make up the lookahead, especially in light of Berwick’s suggestion [5] that lookahead be limited to a clause. The length of clauses measured in either lexical items or chunks is clearly not fixed, but using more highly structured items seems a more promising approach. 12 The requirement that each element be licensed of course follows from the Principle of Full Interpretation.

13 Note that the requirement that each quantifier-like element bind something is related to, but distinct from a prohibition against free variables. 14 Although the parser does not at present concern itself with intermediate traces, their construction could be accommodatedstraightforwardly using this mechanism for chain construction.

CP

In a language such as Chinese, in which wh-phrases are not moved overtly, a parser adhering to the assumptions outlined runs into seemingly insurmountable difficulties. Consider the following examples of direct questions in Chinese; note that “shenme”, the question word corresponding to the English “what”, appears in its D-Structure position in all the sentences. (12) a. Ni kanjian le shenme? you see what “What did you see?” b. Ni shuo ni kanjian le shenme? you say you see ASP what “What did you say you saw?” c. Ni renwei ni shuo ni kanjian le shenme? you think you say you see ASP what “What did you think you said you saw?”

shenme

C’

IP

C

I’

Ni

I

VP

V’

V

“Shenme” can be arbitrarily far away from the position kanjian le trace where it should appear at LF, the matrix clause specifier of CP — the progression in (a) through (c) above shows how multiple embedded clauses can intervene between the Figure 5: Proposed Chinese LF representation wh-phrase and its required LF position. This implies that the parser has no way of knowing that it is dealing with a wh construction until the last word is processed. Given the I propose that they have the following empty-operator standard analysis of wh movement, the parser cannot build analysis, this structure in a left to right fashion, without resorting to unlimited lookahead or backtracking. (14) [CP [C0 [IP Bill du ti ] de proi ]] The solution to this problem is to assume that covert move- The parsing difficulties of these constructions should be ment is rightward [12]. The LF representation which the compounded as the “filler” (proi) is not overt. However, parser recovers is shown in figure 5. the “gap” is identifiable from thematic/subcategorization 16 Under this analysis, Chinese wh-movement can be handled considerations. Thus, the parser handles this the same in a filler-driven manner. The construction of a chain is way as wh-movement in Chinese: the empty operator is triggered by the identification of a wh-phrase, but instead assumed to occupy the position of the trace at S-Structure: of creating a trace and placing it in a movement store, the co-indexed trace just created is left behind while the filler itself is placed in the movement store.

(15) [CP : : : pro : : : de ]

The parser identifies the operator, and moves it to the right Hence, there are two distinct types of filler-driven parsing. to the position it occupies at LF. Hence, this does not pose The first, exemplified in the case of English wh-movement, any difficulties in practice. is filler-driven gap-locating. The second, exemplified by English has a variety of empty operator constructions. In Chinese, is filler-driven gap-creating parsing. her thesis, Browning [7] proposes a comprehensive analysis of constructions such as infinitival relative clauses, adjectival complements, degree clauses, clefts, and var6 Empty Operators ious types of purposive clauses. These are all analysed 16 Chinese also

Consider now Chinese relative clauses. (13) Bill du de zheben shu. Bill read DE15 this book. [NP this book (that) Bill read ] 15 There is no consensus about what this element should be translated as; it is generally assumed to occupy the complementizer position.

allows relative clauses such as

wo lai de shihou I come DE time [NP the time I came ] which involve relativization from an adjunct position. Adjunct gaps in general pose a large problem for licensing parsers, as they do not seem to be licensed in any way. I note the problem, offering no solution at this time.

(20) a. [CP proi [C0 Ci [IP PRO [I0 [VP : : : ti ]]]]] as involving empty operators. I wish here to concentrate on a subset of these constructions, one which seems to b. [IP PRO [I0 [VP : : : ]]] pose significant processing difficulties under Browning’s What is important to note is that the structures proposed analysis. differ significantly at the front of the clause. The point at I will be concerned with infinitival relative clauses as which the ambiguity between a subject-gap and an objectwell as purpose clauses. Both of these clause types have gap infinitival relative clause can be resolved is the object subject-gap and object-gap varieties, and the analysis is position of the clause. As examples (16) and (17) demonthe same for both types of subject-gap clauses and for strate, this point of resolution can be embedded arbitrarily both types of object-gap clauses. For the sake of simplic- far from the front of the clause. If Browning is correct in ity, I will in what follows speak only of infinitival relative her analysis of these constructions, then it is expected that either the subject-gap or object-gap variety cause severe clauses. processing difficulties. The parser, not knowing what type Infinitival relative clauses (those in which the relative of clause it is dealing with, will have to adopt a particular clause is untensed) can be of either the object-gap or the strategy — always assuming that it is dealing with one or subject-gap variety, as shown below. the other type of clause. If it assumes that it is dealing with a subject-gap clause, it will be mistaken if it is in (16) a. Ian is the man to watch. fact dealing with an object-gap clause, and vice versa. If b. Ian is the man to watch Ann. it is incorrect, it will have to backtrack to the front of the (17) a. Ian is the man to tell the police to watch. clause, dismantle the incorrect structure, and build the alb. Ian is the man to tell the police to watch Ann. ternate structure. Backtracking is a very costly operation, The relationship between the predicate (the relative clause) as evidenced by well-known examples, assumed to require and its antecedent (the DP which the relative clause is backtracking, such as, modifying) is that of predication. In contrast to the case of (21) The horse raced past the barn fell. a tensed relative clause, there is no overt relative operator Neither the subject-gap nor the object-gap clause seems to here. cause this severe processing difficulties.17 On this basis I The structure of the subject-gap clause is: reject Browning’s analysis. (18) [DP ]i [IP PROi : : : ]i

Given the availability of both gap-locating and gapcreating processes, a possible solution presents itself. PeThe indices indicate the predication relationship. setsky [26] proposed that PRO may appear on the rightIn the case of an object-gap clause, there is a variable hand side of the projection; I propose to extend this to the which must be bound by an operator. Thus, the basic case of any non-Case marked specifier. Hence, I take it that Case marking imposes a linear order [30] on the tree structure will be: geometry, and that in its absence, there is nothing forcing a specifier to remain on the left.18 (19) [DP ]i [CP Oi : : : ti ]i Browning assumes that “a subject-predicate relationship is licensed if (a) the subject discharges the external -role of the predicate, or (b) the subject agrees with a chain contained in the predicate.” [pg. 62], and that there is a non-ambiguity constraint on predication. This constraint merely states that exactly one of the two clauses (a) and (b) may hold (ie: both cannot hold at once).

Let us at this point take a meta-level view of the proposal. Suppose the parser builds the same structure at the start of the clause in both cases: an empty operator. In both the object-gap and the subject-gap case, we will have an operator in the specifier of CP. In the object-gap case, there are two empty categories further down in the clause, while in the subject-gap case, there is only one. Thus we have:

(22) a. [CP Oi [IP [VP [V0 V eci ] : : : ec : : : Returning for a moment to purpose clauses, Browning b. [CP Oi [IP [VP [V0 V ] : : : eci : : : argues that subject-gap and object-gap varieties act very much alike, and notes that they both occur with a restricted With this analysis, the problem of parsing these structures class of verbs. Furthermore, the class of verbs which allow is basically reduced to that of parsing any A-type movesubject-gap is a subset of the class which allows object- ment. The parser identifies the empty operator, recognizes gap clauses. Despite this, the structures which Browning 17 There is to my knowledge no experimental evidence to bear on this proposes for these two types of purpose clauses (and infini- issue. An experiment to test exactly this is in the planning stages at the tival relatives) are quite different (the first is the object-gap moment. 18 If PRO is Case marked, then it could be argued that the linear case, the second that of the subject-gap structure): ordering is relevant only for phonetically non-null elements.

that it must have originated in an A-position, and begins constructing an A-chain. The parser continues, now on the lookout for a suitable landing site for the displaced operator. In the case of the object-gap clause, such a position is found in the sister to V position (the object gap position). There is no overt evidence for there being anything in the specifier position of either IP and VP; these are thus flipped over to the right-hand side of the projection. We now have: (23) [CP Oi [IP [VP [V0 V eci

:::

subject-position is governed, PROarb must move to the specifier of IP position.19 In the end, the parser builds the following structure, (25) a. [CP proi [IP [VP [V0 V ti ] tj ] PROarbj ] : : : ] b. [CP proi [IP [VP [V0 V ] : : : ti ] : : : ]] The parser constructs the proper structure for both subjectgap and object-gap clauses with no extra machinery. The difference between the two simply falls out as a result of the interaction of two linguistic principles, the EPP and the constraint against vacuous quantification.

At this point the parser must concern itself with the specifier of VP position again (following [18, 29], I take this to be the internal subject position). The EPP requires 7 Conclusion the presence of a subject, and so the parser must insert an appropriate empty category (since there isn’t a subject phonetically present in the input). The details of what The strategy of this paper has been to exploit a variety of constraints to guide the construction of a particular parsing empty category should be inserted are discussed below. scheme. The resultant parsing scheme can recover an LF The case of a subject-gap clause proceeds in a parallel representation for both English and Chinese input in a manner. The parser again recognizes the presence of an single pass. This has been a two-way process, whereby empty operator, and starts to construct an A-chain. What linguistic and psycholinguistic constraints have shaped the will the landing site of the trace of the empty operator be parsing scheme, which in turn has motivated revisions of in this case? As the parser proceeds, the specifiers of both the linguistic theory, making it more precise. IP and VP are again flipped over to the right-hand sides of the projections. There is no gap within V0 . Again, The possibility of parsing typologically diverse languages when the internal subject position is encountered (for the such as English and Chinese using the same parsing second time) the EPP must be satisfied. In contrast to the scheme argues against the view of Mazuka and Lust [25] object-gap case, there is now something to deposit into that this is inherently impossible. It also argues against this position, namely the trace of the A-movement. We approaches to parsing which stress the necessity to develop language-specific parsing schemes, as proposed in thus get: Lee, Chien, Lin, Huang, and Chen [22]. (24) [CP Oi [IP [VP [V0 V ] ti : : : The general approach is appealing, as it seeks to divide the I follow Browning in assuming that the empty operator is task of capturing the linguistic facts expressed in the data pro, though nothing in the analysis hinges on this assump- amongst the linguistic theory (competence) and the parstion. This pro is coindexed with and binds a trace either in ing scheme (performance), thereby hopefully conquering the object position or the subject position of the VP. The some small part of the problem of natural language proPrinciple of Full Interpretation (FI) prevents the insertion cessing. This should be welcome since it simplifies the of an arbitrary empty category simply to avoid a violation construction of robust natural language software tools opof the prohibition against vacuous quantification. In both erating in diverse linguistic environments. of the above cases, the empty operator binds an element which is required for independent reasons. The empty category in the object-gap clause is required to fulfill the 8 Appendix subcategorization requirements of the verb. In the subjectgap clause, the EPP requires the presence of a subject, thus This appendix gives a short sketch of the organization of licensing the appearance of an empty category. the parser. For more details of the basic design, see [4]. The EPP also licenses (and requires) the presence of the The main data structures of the parser are a buffer, a main PRO in the object-gap clause. Since this PRO is not bound stack, and several movement stacks. The buffer is a Firstby any element, it receives “arbitrary” interpretation. Note In First-Out (FIFO) structure. Elements enter the buffer that this PRO could not be bound by the operator as well; 19 If PRO must be Case marked then PRO arb must move to the specifier it would have to be parasitic in some sense, but this would of IP position to be assigned Case by I. The consequences (in terms of violate the anti c-command constraint on parasitic gaps. where PROarb ends up) are the same under the old and the new versions Since PRO must not governed, and since the VP-internal of the theory.

[c2 [d2 ec [d1 d = ec [n2 ec [n1 n = who ec]]]] [c1 c = do [i2 [d2 ec [d1 d = ec [n2 ec [n1 n = you ec]]]] [i1 i = past [v2 [d2 trace ] [v1 v = hit [d2 trace ]]]]]]]

[c2 [c1 [i2 [d2 ec [d1 d = ec [n2 ec [n1 n = ni ec]]]] [i1 i = aspect [v2 ec [v1 v = da_le [d2 trace c = ec] [d2 ec [d1 d = ec [n2 ec [n1 n = shui ec]]]]]

who did you hit.

ni da_le shui.

Figure 6: Parser output for English wh-question

]]]]]

Figure 7: Parser output for Chinese wh-question

References from the rear, and leave it from the front. The parser tries to always have at least two elements in the buffer to satisfy the lookahead requirements of the attacher. The main stack is a Last-In First-Out (LIFO) structure, and is used to store parse-tree fragments during the parse. The parser makes use of three auxiliary data structures for moved elements. These data structures are stacks, though pushing of new elements is restricted to left branching structures, due to the linear nature of the parse. Their growth is thus severely constrained. The three stacks are meant to capture the essence of Rizzi’s Relativized Minimality [27].

[1] Steven Abney. Licensing and parsing. In Joyce McDonough and Bernadette Plunkett, editors, Proceedings of NELS 17, Amherst, Massachusetts, 1987. University of Massachusetts at Amherst. [2] Steven Abney. Parsing by chunks. In Robert C. Berwick, Steven P. Abney, and Carol Tenny, editors, Principle-based Parsing: Computation and Psycholinguistics. Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991. [3] Steven Abney and Jennifer Cole. A governmentbinding parser. In S. Berman, J.-W. Choe, and McDonough, editors, Proceedings of NELS 16, Amherst, Massachusetts, 1986. University of Massachusetts at Amherst.

The parser proceeds by cycling through a sequence of possible operations. The chunker is always activated if there are fewer than two chunks in the buffer. If it is possible, the chunker consumes enough input from the input stream to deposit at least two chunks into the buffer. When there are at least two chunks in the buffer, certain special cases are considered (such as the conditions under which head movement or predication take place). After this, the attacher takes over and considers the top element of the (main) stack, the first element of the buffer, and the contents of the movement stacks. It will attach parse-tree fragments together according to its licensing conditions. This procedure continues until there is no more input in the input stream.

[4] C. Alphonce. Towards an explanatory division of competence and performance: A languageindependent parsing scheme. Master’s thesis, Univeristy of British Columbia, 1992.

Figure 6 gives and example of the parser output for an English wh-question; figure 7 shows the output for a Chinese wh-question (note how the wh-phrase is moved to the right).

[6] Robert C. Berwick and Amy S. Weinberg. The Grammatical Basis of Linguistic Performance: Language Use and Acquisition. Current Studies in Linguistics. The MIT Press, 1984.

[5] Robert C. Berwick. A deterministic parser with broad coverage. In Alan Bundy, editor, Proceedings of the eighth International Joint Conference on Artificial Intelligence, 1983.

[7] M. Browning. Null Operator Constructions. PhD thesis, MIT, 1987. [8] Noam Chomsky. Barriers. Linguistic Inquiry Monographs 13. The MIT Press, 1986.

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