The authors collect lexical a module of English syntactic in the context of a bilingual project. The computer usable version of OA/JD (Hornby, 1974) is used as the primary source. The main focus is on the structure and derivation of valency frames for verbal entries in the target lexicon. Illustration of the complex relation between OALD's verb subc~tegorization codes and the target complementation paradigms is provided, and an approach to the derivation procedure design suggested. 1. INTROD\[b'~ION The present paper describes a part of a larger project, which should result in the extraction of lexical and structural correspondences between grammatical units in large parallel English and CVzech texts. The cortespondenees will then be used to build a transfer module for an English-to-Czech (and possibly Czech-to-English) machine translation system, Final as well as partial results should also be useful as source data for text-oriented lir~uistic research, both hi- and monolingual I . This task entails the need for tools to analyse unrestricted Czech and English texts. In the first stage of the project the goal is to produce Czech and English lexicons of adequate coverage and implemented analysis grammars, which will later be augmented with tools for preliminary disambiguation. The parser will build annotated dependency structures, usable for tagging word forms, clauses and sentences with morphological and
ACRES DE COLING-92. NANTES, 23-28 AOOT 1992 5 5 3
Universita Karlova Praha, Czechoslovakia data for analysis research syntactic information. The lexicon and grammars, enriched by feedback from the parsed texts, can later be used within t}~e machine translation system proper.
At present, the pril~ry source of lexical data for the English analysis is a m~chine readable dictionary, preprocessed to contain only relevant information in a transparent format. This paper foeusses on how valency frames for verbal entries are extra~ted from subcategorization codes in the ,~chine readable dictionary. 2. ~ CI{OICES
Even though the correspondences between parallel text units can be established at an arbitrary level starting from word forms up to an elaborate logical representation, the practical solution seems to lie somewhere in between. The approach we have chosen is based on the representation of linguistic analysis in terms of underlyin~ (tectogrammetical) structures, which are determined by the given laaqgu~ge, but void of various irregularities of the surface strings, including the ~unbiguity of n~rp~mic and surface syntactic units. 2 A "deeper" analysis would increase the. risk of errors and introduce more theoretical bias while a very shallow level would require larger amounts of data to arrive at simple facts when parallel text units are compared.
The (underlying) syntactic description is dependency-based (with coordination and apposition as relations of a different type) and the project described here makes it PROC. OF COLING-92, NANTES, AUG. 23-28, 1992 possible (i) assumptions of data collection, an implementable surface string representation. to test the basic the theory on a large and (ii) to formulate
relation between the
and the underlying
A constrained-based (unification) formalism was selected due to its declarativeness, conciseness and formal rigour, but its other interesting properties were a\]so appreciated: i.a., the important role of the lexicon and the need I~ treat surface facts within the same rigoro~ framework a.q deeper concep~. 3 3. THE SOLACE
As a shortcut towards a lexicon of reasonable coverage we decided to build upon an available machine readable dictionary, which we intend to augment later by hand and from other sources. Our primary source of English lexical data is now CUVOALD, or the Expanded Computer Usable Version of the Oxford Advanced Learner's Dictionary of Cul.rent English, 3rd edition (OALD, Hornby, 1974), w}Kch is available from Oxford Text Archive (see Mitten, 1986) 4 . CUVOALDIists all headwords, headword variants and derivatives with simple codes denoting word classes and inflection patterns, supplemented by verb pattern codes for verbs. Sense distinctions from OALD are not retained.
Where~q the derivation of lexical information as needed by the analysis from CUVOALD word class codes is relatively straightforward, the OALD verb pattern codes, which are crucial for our purpose, present a real challenge. The dictionary classifies verbs according to the number and form of complements in%~ 51 "verb patterns", marked by numbers 1-25, supplemented in some cases by letters (4A,4B,4C,4D,4F). The number of verbs in a single pattern is quite variable: starting from a single item in \[VP4F\] for he followed by an infinitive up to 4855 standard transitive verbs in \[VP6A\]. A pattern groups together verbs which exhibit the same behaviour in a standard context and are subject to the same set of transfor~tions ACl~.S DE COLING-92, NANTES, 23-28 AOUT 1992 5 S 4 under specified conditions. So e.g. the class of intransitive verbs \[ VP2A\] can take introductory there and postpone the subject if it is indefinite and "heavy" : There comes a time when we feel we must make a protest. A single pattern used for verbs which allow mot phesyntactic variations a complement. ( \[VPI\] : She's good heal th/here/a pretty A different verb pattern is, used if only a subset of the class perndLs the variation. She en~oys swimmir~g / *to \[VP6D\] : She likes swim~ting / Some vat iatio~kq may be treated as a different verb pattern. (This is the case of the above example: She likes swimming. \[VP6D| and Site likes to swim. \[VPTA\] ) Akkerman (1989) lists several shortcomings of the OALD verb patterns. As Sampson (1990) noted, some of them are arguable. For our purpose, the most proble,~tic seems to be the treatment of compound verbs ( with the resulting loss of infor.~tion in CUVOALD) and too surface-level definition of some verb patterns. These classes are quite a heterogenous collection: by \[VPI4\] are marked verbs in all of the following uses, the only requirement being that the verb is followed by a noun and a prepositional phrase: They accused him of stealing the book. I explained my difficulty to him. Compare the copy with the original. Another "misbehaved" where, depending on infinitive can be adjunct: pattern is VP4A the verb, the
complement or The ~winm~er failed to reach the shore. lie came to see that he was mistaken. She stood up to see better. Apart from these "systemic" blemishes we expect a number of other inconsistencies and errors to appear during t}~ process of derivation and use of the target lexicon. PROC. OF COLING-92, NANTES, AUG. 23-28. 1992 is also the same
of
dark~in
girl. ) however, relevant (\[VP6C\]: swim. vs. to swim.) 4. "lltE TAIK~T
The target lexicon conta:imq the fol\] owitN information about 'the valency of a verb ( or its complementation), grouped in an entry as a comp\]ementation t~radigm: SUBCATFC~)RIZATION LIST (SC) gives syntactic and merphologica\] categories for every del~endent, i .e. ei ther a particil~nt ( comp\] emenh, n~y be ob\]igatory or optional) or an ohZigatory free modificaLion ( obligatory adjunct). An item in the list is Jn fact an tmderspecified representation of the correspondirlg dependent. The ordering of items in the list corres1~nds t~ the unmarked word order in a declarative sentence. SYRrI'ACTIC FIULME (SF) , a feature structtme with syntactic functiorks as attributes; values of these attributes are co-indexed with the corresponding ite.~ of the subcateKorization list. UNDERLYING STRUCUVRE (IJS), a feature structure with tectogranm~tical functions ~s attributes; valucs of these attributes are identical w:ith ImderZyiri~ structures within the correspond:ing items of sul~aLeKorization list and syntactic frame. The value of the attribute C~DV (governor) is identical with the value of the lexeme attribut>e of the verb's feature structure.
The armlysis will establish index \] inks between saturated fra.m s\] ors and their fillers in the analysis tree. This will provide e~y access i~ the analysis resu\] ~ at the three \] eve\] s of description, hig}~ ighting the structure of the sentent, Ja\[ (:ore,
The 1hi\]owing simple example gives comp\] enmntation paradigm for an intransitive verb. N\[ nora| is shortI~nd for a feature structure representing noun in the noEu.t~tive c~k~e with saturated subcategorization requirements ; the numbers co-index feature structures which are s~red as values of some attributes, the ,a.a I 1 index selectq only a part of the structure, namely the nominal equivalent of the underlyin~ structure; the attrJ bute GOV g~ yes the lexica\] value of the verb while ACP stands for ACRES DE COLING-92. NAN*rEs, 23-28 Ao\[rr 1992 S 5 5 actor/bearer, t|m function represen~ ring subject of an active verb at the underlying level. Angle bracket~ enclose lists, square brackets (con~ .juctio~ of) feature structures, curly brackets disjunctions. Commas separate members of conjuction, vertical bars members of disjunction. SC < sF \[ US \[ \[ll Nlnomlz > , sunJ \[11 \] , (~)V sleep , ACi' 12\] \] The sa,~ could \[~ expressed in a PATR-Iike style (Shieber (1986) ) : < SC < SC < S\[; < US < US first > = N\[nom\] rest > = end SUBJ > = < SC first > C/J)V > = sleep AC'F > = < ~7 first US >
Next, we Rive two possible complementation paradigms for a transitive verb. ( PAT stands for patient, VI prespart, SC<N3 > \] 4 is abbreviation for present participle form of a verb whose single va/ency slot for subject in the SC list is co-indexed with the ~ct~r/bearer of the n~trJx verb): SC SF US SC SF US < I1\] N\[nomla , 121 N\[acc\]4 > , \[ s\[~u 111 , OBJ \[21 I , \[ C<)V enjoy , ACt 131 , PAT \[4\] \] < \[I\] N\[nom\]a ,
\[2\] V\[prespart,SC<N3>\]4 > , \[ S\[~J \[i\] , ()IkJ \[2\] \] , \[ GOV enjoy , AUI' \[3\] , PAT \[4\] \]
&q the value of the attribute PAT of er~joy i.s shared with the value of the attribute US of the object, the correct va\]ue for the dependent verb's ACt attribute is supp\] led via co-indexing of the subject of enjoy wJ th the subject of the non- finite clause within the SC list of er~joy: US \[ GOV enjoy , ACt' \[31 ,
PAT \[ GOV swim , AC£ \[3\] \] \]
The complementation paradigm, rat}mr tI~n being stated within full-fledged feature structures, is expressed in terms of templates, preferrab\] y allowing defaults and multiple inheritance, Accordingly, the above two paradig~ wil\] be expressed as follows : PROC. OF COLING-92, NAbrrEs, AUo. 23-28, 1992 transi tive transitive , 2ing , equi
Two verbal entries can he related by a lexical rule with the effect that one of these two entries need not be explicitly present (the ot}~er should then be ~rked by the rule's name). This will solve phenomena such as there preposir~, dative alternation, and passivization.
The collection of three "levels" of description within a single complementation paradigm provides a means to express rather subtle differences. Let us take as an example four superficially identical constructions: (a) I (b) (c) (d) I I I w~nted him to see the monster. expected him to see the monster. elected him to see the monster. told him to see the monster.
Following Quirk et al. (1985, p.1216), the verb is monotransitive in (a), complex-transitive in (b) and (c), and ditransitive in (d). The example (b) is closer to the monotransitive type while (c) is closer to the ditransitive type. If we have the subcategorization list SC < \[1\] N\[nom\]4 , \[2\] N\[acc\]5 ,
\[3\] V\[inf, SC<Ns>\]6 > to express the superficial identity of all the four cases, we can assume the above verbs to have the following syntactic frames: (a) (b) (c) (d) SF \[SUBJ\[I\], OBJ\[3\]\] SF \[SUBJ\[I\], OBJI2\], OBJCOMPL\[3\]\] SF \[SUBJ\[1\], OBJ\[2\], OBJCOMPL\[3\]\] SF \[SUBJ\[I\], OBJ\[3\], OBJ212\]\] The difference between the types (a) and (b) vs. (c) and (d) is that between the Raising and Equi types. Therefore, (b) will have only two participants at the level of underlying structure while (c) will have three: (a) (b) (c) (d) US \[ACr 14\], PAT \[6\]\] US \[ACT \[4\], PAT \[6\]\] US \[ACT \[4\], PAT \[5\], EFF \[6\]\] US \[ACT \[4\], PAT \[6\], ADDR \[5\]\] ACRES DE COLING-92, NANTES, 23-28 AO't3"r 1992 5 5 6 The respective templates will be: (a) (b) (c) (d} transitive, 3inf , complex-transitive, complex-transitive, di trensi tive, 3inf raising
3inf
3inf , equi , raisin~ , equi
A problem remains bow to derive such information from OAL/T s verb patterns.
CUVOALD was not pri.~rily intended for use with a syntactic parser, so a few modifications were necessary. First, the pronunciation field was deleted and homograph entries with different pronunciations r~erged. ( In CUVOALD, each word, or word form, has only one entry, unless it has two different pronunciations. ) Second, entries headed by regular forms within irregular paradigms as headwords were also deleted. And finally, reference to base forms was provided in entries of all the remaining nonbase (irregular) forms. Base forms of irregular paradigms were marked by a code specifying the paradigm type. After that, we tried to find a way how to derive the complementation paradigms.
Ideally, templates o f the sort described in Section 4 should Correspond to OALD verb patterns while lexical rules would account for structures listed in Hornby (1975) as variants of the same verb pattern. Although this idea works in t}~e case of the most frequent patterns ( \[VP2A\] , \[VP6A\] ) , there are many patterns where the relation between pattern and paradigm can be l:n, n:l, or even n:n (n > I) (see Section 3).
The case of n patterns : i paradigm reduces the number of paradigms and as such is a welcome situation. The case of 1:n can mean (i) ambiguity for all verbs listed under the pattern (and c~n possibly be accounted for by lexical rules) , (ii) the possibility to subdivide the verbs of this class into n subclasses, or (iii) a combination of the two. For (il, the derivation of complementation paradi~l from a verb pattern will yield a disjunction. For (ii) , verbs with different complemenration paradigms should be distinPROC. OF COLING-92, NANTES, AUG. 23-28, 1992 guished. Boguraev and Briscoe (1.989) used valency codes in Ii)0CE (Lon~gnan Dictior~ary of Contemporary English) to autematical\]y extract the (explicitly unmarked) distinction between Equi and Raising verbs. Similar approach can be tNed to ,~ke this and other distinctions in OALD by taking into account co-occurences of verb pat terra. (Am situation is simpler ira that we, ~s yet, make no attempt to treat distinct. word senses, and more difficult in tidal the blurred se~e distinctions can have negative effect on any derivation procedure. It renmir~ to be seen whether such a n~thod w:i\]\] lead t~ resul~ of sufficient reliability. However, at the same time we have fx) supply n~re inforn~tion t~ some classes of verbs, for which any pessibilJty of autonmt~c treatment is ex\[uded. The current efforts include the specification of lexica\] values of particles and prepositions for compound verbs and assigning verbs n~rked by verb pattern codes such m~ VP14 to relevant subclasses.
The correspondences between the OALD patterns and complementation paradigms are stated in the simple cases by rules relating one or more patterns to one or more paradigms - templates. Where possible, frequently co-occurring verb patterns are collapsed into a single paradigm with local disjuction, e.g. \[VP61)\] and \[VPTA\] for like (swimming/ to swim) give the following template: \[ transitive, { 2ing / 2inf }, equi \], which expands into: SC SF US < \[I\] N\[nomb ,
\[ 21 V\[{prespartl inf} ,SC<N3>\]4 > , \[ suBJ \[11 , OBJ lZl \] , \[ GOV like , ACI' \[3\] , PAT \[4\] \]
Now t~re are two possible strategies representing two extremes. The first strategy disregards the actual distribution of verb patterns in the dictionary and atten~ts to combine results of rule application into a compact and meaningful complementaLion paradigm. The second strategy starts from a list of all combinations of verb patterns within the dictionary and assigns a rule to every combination. Let us look }mw the first approach works. AcrEs DE COLING-92, NAtUrES, 23-28 aotn" 1992 5 5 7 PROC. OF COLING-92, NANTES, AUO. 23-28, 1992
The process of derivation of a complementation paradigm for a verb entry consists of the following steps: 1. Application of rules rewriting a verb pattern code (or more verb pattern codes if the resulting paradigms can be related by a lexical rule) by a template or a sequence of templates collected by logical operators "and" and "or", the result may be marked by one or mere lexical rule names. Rules rewriting n~re l~tter~ are preferred to those rewriting fewer i~tterns. A rule may be, supplemented by a condition stipulating the presence or absence of' other paradigi~ within the same entry. A rule whose condition is satisfied is preferred to a rule wJ thout condition. Verbs with patter~ which do not, corresl~nd to a single complementation i~radigm while cooccurring verb patterns do not indicate a preference ibr one paradigm or the other have to be treated ,~nu~lly. 2. Simplification of the sequence of templates by making all disjunctions as local as possible. 3. Consistency check performed by expansion of the sequence of templates into feature structures. E.g.: l~lieve 3A 6A 9 10 25 step 1: rules applied: 3A -> transitive, prepositional 6A -> transitive, 2n 9 -) trans'itive, 2cJs, 2that i0 -> transitive, 2cls, 2wh25 -> complex tran~itive, 3Jnf,
rai sing
/ ^ { 12A: 12B: 12C', 13A: 13B } a) after application of the rules: { transi tive, 2prep ', b )
tra~gitive, 2n ',
trar~itive, 2cis,
transitive, 2cls,
complex transitive,
after step 2: { transitive, { 2prep '~
2n',
2cls, { 2that', ~h-} } I
complex transitive, 3inf, raising } after step 3: US \[ ACT\[3\], PAT\[4\] l, SF \[ SUBJ!I\], OBJ\[2\] \], SC < \[11 N\[nom\]3,
\[2\] { N\[ { prep', acc} \] ',
V\[ cls,{ that: wh} \] }4 > US \[ ACT\[4\], PAT\[6\] \], SF \[ SUBJ\[I\], OBJ\[2I, OBJCOMPL\[3\] \], SC < \[I\] N\[nom\]4 , \[2\] N\[acc\]5 ,
\[ 3\] V\[ inf, SC<N5> )6 > a) This is a condition stipulating that neither of the pat terrL~ should be present; the character ^ stands for negation. b) This is the template of a prepositional verb. The lexical value of the preposition should be supplied.
This looks like a principled solution, but step i can be a source of unforeseen complexities with the result that too ,~ny entries will have to he handled manually. The second strategy is much safer: if there are not too many different combinations of verb patterns it might not be too difficult to state rewriting rules for all of them, thus e\]iminati,~ steps 2 and 3 from the above procedure. However, to make a decision, some statistical analysis is necessary.
CUVOALD lists 5695 verbs with 633 different combinatioi~ of verb patterns. ~ 4853 verbs (85.2%) are marked by one of the 56 most frequent combinations (each occurring seven and more times). The first ten most frequent combinations are given below: verb patterns frequency 6A 2A,6A 2A 6A,14 2A,2C 2A,3A 6A,15B 2A,2C,6A 2A,2C,6A,15B 3A At the other end, combinations occurring two and more times, 119 three times and 77 five and mere times. AcrEs DE COLING-92, NANTES, 23-28 AOt)r 1992 5 5 8
also seems for combinathe remaining ar~ perhaps according to together form 1971 575 338 331 165 137 101 I00
81
64 t~re only are 442 once, 191 and more
Another survey was aimed at finding most frequent combinations as proper subsets of the full combinations treated above. E.g. the combination of three patterns 2A,3A,6A occurs alone in 54 entries, but as a proper subset of a larger combination already in 566 entries.
From the above data it seems that a compromise between the treatment of individual verb patterns and of entire combinations would be most efficient. 119 combinatio~ can already be treated by individual rules quite comfortably while the rest can be composed from results of rules applied independently, where more alert supervision is required. It feasible to use the rules tions to treat parts of lists of verb patterns, add a few more, se#ected the second statistics. 6. P~SPM\[rr\]A~
Lexicon and gram,~r the basis for the extraction of lexical and structural correspondences. Other tools are necessary, however, and we are currently designing specifications for such tools.
Besides the non-trivial task of text cleanup, for which no special tools will be used, two major needs remain: text unit align,~nt and data extraction methods.
Automatic text unit alignment (on word, phrase, and sentence level ) is also non-trivia/. On the sentence level, we will employ a method for al igpmlent based on sentence length ( Gale 1991 ), for which we have developed a f\]exible front-end for recognizing sentence houndaries. We are considering an extension of Church's algorithm taking into account lexicon-based elementary word correspendences (as in Kay (1988) and Catizone et al. (1991) ) for better accuracy, but this extension has not been implemented yet.
Methods for data extraction are still under development. However, it is clear what such data should look like. As our output representation is PROC, OF COLING-92, NANTES, AUo. 2.3-28, 1992 far from the inter\]Jr~u~ ideal, the data will basically be transfer data in a form fitti~ the structural transfer model, following the ideas of Kaplan et, a.L. ( 1989). The actual implementation, however, will follow the l~.ttern of the transfer module in the experiment~l machine trans\]at\]on system EL\[\] (Russel et a\] . (1991) ) . NOT~
This MAchine Eng\[ J sh, carried Initiative
it is discuss toire of valency relatio~ and their classJ fication. The interested reader can find a detailed analysis of these issues and a comparison with other theories of deep (underlying) structure Jn Sgall, HajJSov{~ and Panevov~ (1986, esp. Ch.2). 3 As we are involved in the development of a practical constraint-based system, we are aware of the necessity to include some centre\] or dynamic information in addition to the static description sup;x)rted by traditional constraint-based for,~lisms. We expect to deal with this issue seriously in later stages of the project, when partia\] results wi\]\] be available. 4 CUVOALD comes in two versions: one lists base forms pl~ a\]\] forms of J rregu\] ar words whJ \]e the other contains all inflected forms explicitly. As we intend t~ }rove a merphological comi~nent, we are u~ing the ba.~e forms version. 5 These and following numbers include base forn~ only, as well as 876 verbs which were not marked by any p~ttern and for which defaults were used: 6A for transitive verbs, 2A for intransitive verbs. Akkerman, E. (1989) "An independent analysis of the LDOCE grammar coding system", J n B. Hoguraev and T. Briscoe (eds.) Computational Lexicography AcrEs DE COL1NG-92, NANTES, 23-28 AoU-r 1992 5 5 9 PROC. OF COL1NG-92, NANTES, AUG. 23-28, 1992 project, ca\]\]ed MAT'RACI,; (from TRAnslation between Czech and
J s one o f the projecL~ out within the IBM Academic
Jn Czechoslovakia. not the aim of this paper to and substantiate the reper-
for Natural Langua\[4e Processing,
Longn~n, London and New York Boguraev, B. and T. Briscoe (1989)
"Utilising the LIX)CE gram,~r codes",
i bi dem (~tizonc, R., }£m~se\]\], G. and S. Warwick ( 199\] ) "Deriving Translation Data from Biliz~ual Texts", in Zernik
(ed.) Lexical Acquisition: Using
on-Jine Resources to Build a Lexicon, Lawrence Erlb~um. Gale, W. A. and K. W. Church (in prep. ) A Program for Aligning Sen-
tences in BilinA'~1 Corpora, submitted to CL (1991). l{ornby, A.S. (1974) Oxford Advanced
Learner's Dictionary of Current Eng-
lish, 3rd edition, Oxford University Press, London. Hornby, A.S. (1975) Guide to Patterns
and Usage in F~\]fflish, 2nd Edition, Oxford University Press, l~)ndon. Kaplan, 1~. M., K. Netter, J. Wedekind and A. Zaenen (1989) "Translation by Structural Correspondences", in Pro-
ceedings of the 4th EACL, ACL, Man~ chester, UK. Kay, M. and M. R6scheisen (1988)
Text-'l~ans'lation AliKrm~ent, unpublished manuscript, Xerox Pale Alto Research Center. Mitton, R. (1986) "A partial dictionary of English in Computer-\[~sable Form" , J n L1 terary and Linguistic
Computing 1:214-215. Quirk, R., S. Greenbaum, G.Leech, J. Svartvik (1985) A Comprehensive Gramnmr of the Ec~lish Language, Longm~n, \]mndon and New York. Russell, G., A. Ballim, D. Estival and S. Warwick-Armstrong (1991) "A Language for the Statement of Binary Relations over Feature Structures", in l~oceedir~s of the 5th EACL, ACt, BerlJ n, Germany. Fm~,p~on, G. (1990) a review of B. Boguraev and Ted Briscoe (edas.) Compt~ rational Lexicography for Natural Language Processing, in Cx~mputational Linguistics 2 : 113-116 Sgall, P., E. Haji~ovh and J. Panevov& (1986) The Meaning of the Sentence in its Semantic and Pray'tactic Aspects (Edited by J. Mey), Reidel, Dordrecht / Academia, Praha. Shieber, S.M. (1986) An introduction
to Unification-Based Approaches Grit. CSLI Irecture Notes No. Stanford: Center for the study Language and Information. to 4, of