Richard K i t t r e d g e , C h a i r p e r s o n
Universitd de Montreal Montreal, Quebec PQH3C3J7
Panelists
Joan Bachenko, Naval Research L a b o r a t o r y Ralph Grishman, New York University Donald E. Walker, SRI International Ralph Weischedel, University of Delaware
5.1 W h y Are Sublanguages Important for Applied Computational Lingustics?
F o u r of the five panels at this workshop are assess- ing the perspectives in applied computational linguis- tics for four important problem areas: natural-language interfaces, machine translation, text generation, and concept extraction. F o r each of these areas, it is as- sumed that any applied system will be oriented toward the particular variety of natural language associated with a single knowledge domain. This follows from the now widely accepted fact that such systems require rather tight, primarily semantic, constraints to obtain a correct analysis, and that such constraints can at pres- ent be stated only for sublanguages, not for the lan- guage as a whole. Although a practical system may well have c o m p o n e n t s that are designed to accommo- date the whole language, it must also anticipate the particular syntactic, lexical, semantic, and discourse properties of the sublanguage in which it will operate.
R e s e a r c h into the linguistic structure of w e a t h e r reports, medical records, and aircraft m a i n t e n a n c e manuals has led to specialized grammars for the sub- languages of these domains. Central to each sublan- guage grammar is a statement of the functionally simi- lar word classes and the c o - o c c u r r e n c e restrictions among these classes. When a parser, g e n e r a t o r , or translation system incorporates such a precise linguis- tic description, it becomes not only more efficient but also capable of discriminating b e t w e e n sentences (and texts) that are a p p r o p r i a t e to the d o m a i n and those that are grammatical but inappropriate. In addition, the word classes used in the grammar, and the hierar- chies relating these classes, are an important part of the knowledge structure for the domain.
5.2 H o w Do S u b l a n g u a g e s A r i s e ?
When natural language is used in a sufficiently restricted setting, we may be justified in calling the resultant forms a sublanguage. Although there is no generally a c c e p t e d definition of this term. Several
factors are usually present when the subset of a natu- ral language is restricted enough for efficient semantic processing.
• Restricted domain of reference. The set of objects and relations to which the linguistic expressions refer is relatively small.
• Restricted purpose and orientation. The relation- ships among the participants in the linguistic ex- change are of a particular type and the purpose of the exchange is oriented towards certain goals. • R e s t r i c t e d m o d e of c o m m u n i c a t i o n . C o m m u n i c a -
tion m a y be spoken or written, but there are constraints on the form of expression, which may include " b a n d w i d t h " limitations. Compressed (or telegraphic) language forms may reflect the time and space constraints of certain c o m m u n i c a t i o n modes.
• C o m m u n i t y of participants sharing specialized knowledge. The best canonical examples of sub- languages are those for which there exists an iden- tifiable community of users who share specialized knowledge and who communicate under restrictions of domain, purpose, and mode by using the sublan- guage. These participants e n f o r c e the special pat- terns of usage and ensure the c o h e r e n c e and com- pleteness of the sublanguage as a linguistic system.
5.3 Constraints and Extensions in the Grammar of a Sublanguage
A typical sublanguage makes use of only a part of the language's lexical, morphological, syntactic, seman- tic, and discourse structures. These restrictions on its grammar, once d e t e c t e d and e n c o d e d in the form of rules, can be exploited during automatic processing by greatly reducing the n u m b e r of possibilities to be con- sidered. A sublanguage may also exhibit structures (and, hence, rules) that are not normally regarded as part of the standard language. In the most general case, then, a sublanguage g r a m m a r intersects, but is not contained in, the grammar of the general or stand- ard language from which it derives.
Some of the typical constraints and extensions found in each c o m p o n e n t of a sublanguage grammar are given below, along with r e f e r e n c e to r e c o g n i z e d techniques for describing the constraints and for iden- tifying them in a corpus of texts, when appropriate. In
Richard Kittredge Sublanguages
addition, we m e n t i o n a n u m b e r of m e c h a n i s m s f o r capturing these constraints for the p u r p o s e s of c o m - puter processing.
5.3.1 Lexical and Morphological Characteristics T h e most o b v i o u s f e a t u r e of a sublanguage is its specialized lexicon. N o t only is the set of w o r d f o r m s (and their possible meanings) highly restricted, but the productive w o r d - f o r m a t i o n rules m a y be of a particular kind, s o m e t i m e s unique to the s u b l a n g u a g e or to a family of related sublanguages. Texts in medicine and p h a r m a c o l o g y , for example, m a y contain a rich v a r i e t y of n a m e s for diseases and drugs, which are c o n s t r u c t e d using c h a r a c t e r i s t i c affixes. Military s u b l a n g u a g e s m a k e f r e q u e n t use of a c r o n y m s which o b e y describa- ble rules of n o u n p h r a s e f o r m a t i o n in the g r a m m a r . M a n y sublanguages e m p l o y symbolic expressions (e.g., m a t h e m a t i c s ) or a b b r e v i a t i o n s which can be shown to have their o w n m o r p h o l o g i c a l characteristics.
Techniques for identifying the special m o r p h o l o g y of s u b l a n g u a g e t e r m s are readily available f r o m lin- guistics. In cases where the lexicon is large, the de- signer of a c o m p u t a t i o n a l s y s t e m m a y find it profitable to include w o r d - f o r m a t i o n rules in a special processing phase.
5.3.2 Syntactic Characteristics
Early w o r k on restricted language has s h o w n that the syntactic description of a naturally occurring sub- language m a y differ significantly f r o m t h a t of a unrestricted language. In the highly c o n s t r a i n e d style of w e a t h e r bulletins, there is little r e s e m b l a n c e b e - tween the syntactic structure of telegraphic f o r e c a s t s and that of general language. T h e syntactic rules are essentially those of a s e m a n t i c g r a m m a r ( B u r t o n 1976). T h e TAUM-METEO system ( C h e v a l i e r et al. 1978) f o r t r a n s l a t i n g C a n a d i a n w e a t h e r bulletins is based on a g r a m m a r arrived at through a distributional analysis of a large corpus of these texts. In less ster- e o t y p e d sublanguages such as medical records, there m a y be b o t h elliptical s e n t e n c e f o r m s and their full- s e n t e n c e p a r a p h r a s e s in the s u b l a n g u a g e . T h u s the NYU s y s t e m for extracting f o r m a t t e d data f r o m medi- cal r e c o r d s (Sager 1978, 1981) m u s t include in its parser special rules for elliptical f o r m s as well as m o r e general syntactic rules for the full forms.
M o s t sublanguages of English o b s e r v e the syntactic p a t t e r n s of standard English but m a y differ m a r k e d l y in the f r e q u e n c y of usage of various c o n s t r u c t i o n s . F o r e x a m p l e , m a n y of the q u e s t i o n f o r m s , stylistic inversions, and exclamatives of c o n v e r s a t i o n a l English are totally absent f r o m technical literature ( L e h r b e r g e r 1981). G r a m m a r s for processing technical language m a y t h e r e f o r e delete the c o r r e s p o n d i n g p r o d u c t i o n rules for analysis in technical domains. O n the o t h e r hand, s o m e sublanguages m a y use syntactic construc-
tions u n k n o w n in the general language, in which case the a p p r o p r i a t e p r o d u c t i o n s m u s t be included in the sublanguage g r a m m a r .
E v e n w h e n certain g r a m m a t i c a l constructions can- not be ruled out of the g r a m m a r , they m a y be of such high or low f r e q u e n c y in the s u b l a n g u a g e t h a t this fact can be used to reorganize the o r d e r in which rules are tried or to change the p r e f e r e n c e weighting assigned to c o m p e t i n g syntactic analyses.
5.3.3 S e m a n t i c C o n s t r a i n t s
T h e r e s t r i c t e d d o m a i n of r e f e r e n c e of a s u b l a n - guage is m i r r o r e d in the w a y words are used with re- spect to o n e a n o t h e r . A d i s t r i b u t i o n a l analysis of w o r d c o - o c c u r r e n c e s in a large corpus of texts ( H a r r i s 1963; H i r s c h m a n , G r i s h m a n , and Sager 1975) allows a c o m p u t a t i o n a l linguist to group words into equivalence classes and to d e s c r i b e the o c c u r r i n g s e n t e n c e s in t e r m s of these classes. C o m p u t a t i o n a l s y s t e m s which use the s e m a n t i c g r a m m a r a p p r o a c h ( B u r t o n 1976) state the syntax directly in t e r m s of such distributional classes, which are r e l e v a n t for the s e m a n t i c or func- tional distinctions to which the s y s t e m s sensitive. Collapsing syntax and semantics in this w a y is useful for small sublanguages ( H e n d r i x et al. 1978; E p s t e i n and Walker 1978), but there is the d i s a d v a n t a g e that the g r a m m a r has no generality and a new one has to be w r i t t e n f o r e a c h n e w s u b l a n g u a g e . T h o u g h o n e a r g u m e n t for s e m a n t i c g r a m m a r s has b e e n that they are c o m p u t a t i o n a l l y m o r e efficient, r e c e n t e x p e r i m e n t s in which a s e m a n t i c g r a m m a r was c o m p a r e d with a linguistically m o t i v a t e d g r a m m a r for the s a m e d a t a b a s e d e m o n s t r a t e d that the latter could be just as efficient
(cf. Sagalowicz 1980).
In m o r e c o m p l e x sublanguages it is usually neces- sary to m a i n t a i n t r a d i t i o n a l s y n t a c t i c c a t e g o r i e s , a n d hence to couch parsing rules in t e r m s of these c a t e g o r - ies. In this case, s e m a n t i c c o n s t r a i n t s in the f o r m of selectional restrictions can be applied either during or directly a f t e r parsing to eliminate those syntactic a n a - lyses that give m e a n i n g s impossible in the s u b l a n g u a g e (Sager a n d G r i s h m a n 1975, Sager 1981, R o b i n s o n 1,980).
M o s t sublanguage texts also h a v e larger i n f o r m a - tion structures b e y o n d the word-class c o - o c c u r r e n c e s of single sentences. An analysis of the i n f o r m a t i o n f o r m a t s of m e d i c a l r e c o r d s ( H i r s c h m a n a n d Sager 1981) has b e e n carried out for the p u r p o s e of infor- m a t i o n retrieval. F r a m e - l i k e structures m a y also be e m p l o y e d to recognize and e x t r a c t larger i n f o r m a t i o n c o m p o n e n t s (e.g., B o b r o w et al. 1977, Schank et al. 1980).
entails the a s s i g n m e n t of p r o p o s i t i o n a l s t r u c t u r e s to text passages ( W a l k e r and H o b b s 1981). D o m a i n and p r o t o c o l analysis (Davis 1977, Newell a n d Simon 1972, M a l h o t r a 1975) provide techniques for h y p o t h - esizing facts and i n f e r e n c e rules a p p r o p r i a t e for se- mantic analysis and reasoning procedures. K n o w l e d g e acquisition p r o c e d u r e s (Davis 1977, H a a s and H e n d r i x 1980, R y c h e n e r 1980), n o w under investigation, could significantly aid in the building of semantic and infer- ence c o m p o n e n t s .
5.3.4 D i s c o u r s e C o n s i d e r a t i o n s
R e c e n t research has s h o w n that the w a y in which s e n t e n c e s are strung t o g e t h e r to f o r m c o h e r e n t text can vary considerably f r o m one sublanguage to anoth- er. In addition to differences in discourse-level se- m a n t i c s t r u c t u r e s (see 5.3.3), s e p a r a t e s u b l a n g u a g e s m a y m a k e d i f f e r e n t use of a l a n g u a g e ' s linguistic m e a n s of textual cohesion. In view of the considera- ble a t t e n t i o n given to a n a p h o r a in the literature of c o m p u t a t i o n a l linguistics, it is worthwhile to note that certain technical sublanguages contain no o c c u r r e n c e s of anaphoric pronouns, while others m a k e use of spe- cial anaphoric devices (Kittredge 1981). E v e n w h e n a technical s u b l a n g u a g e uses p r o n o m i n a l a n a p h o r a , it often a p p e a r s that the sublanguage effectively restricts it to cases where the a n t e c e d e n t noun phrase occurs in the preceding sentence or even in an earlier clause in the same sentence. Needless to say, the s t r a t e g y e m - ployed for establishing c o - r e f e r e n c e in a sublanguage must therefore take into a c c o u n t the b e h a v i o r of each a n a p h o r i c device in that same sublanguage. In m a n y cases, a far simpler algorithm can be used t h a n would be necessary for unrestricted language. In a n y given language, the s e m a n t i c c o h e r e n c e and g r a m m a t i c a l c o h e s i o n of a text can be signalled b y a v a r i e t y of linking devices. F r o m a language's i n v e n t o r y of de- vices, each sublanguage seems to m a k e a r a t h e r dis- tinctive and limited selection. Stock m a r k e t r e p o r t s avoid repetition of the same v e r b in successive sen- tences, using s y n o n y m s instead, w h e r e a s technical manuals a p p a r e n t l y avoid s y n o n y m y at the e x p e n s e of lexical repetition (Kittredge 1981). T h e use of tense or tense v a r i a t i o n m a y also fit a distinctive p a t t e r n . All such tendencies, w h e t h e r probabilistic or absolute, m a y be exploited during the design of o p t i m i z e d sub- language processing systems.
5.4 Factors Defining S u i t a b l e C a n d i d a t e A p p l i c a t i o n s
The sublanguage a p p r o a c h to language processing m a y not be a p p r o p r i a t e to all varieties of restricted language or all types of application. It m a y only be profitable where there exists an established group of users who help to identify and define the knowledge domain. In addition, the domain should be relatively well-defined and internally consistent. The m o s t tract-
able s u b l a n g u a g e s f r o m the c o m p u t a t i o n a l point of view are those that p r e s e n t a simple discourse struc- ture. Finally, each application should be one in which the c o m p u t e r is an a p p r o p r i a t e m e d i u m of c o m m u n i c a - tion or processing (e.g., s p o k e n sublanguages or ones for which p e r m a n e n t records would not or should not be kept m a y not be a p p r o p r i a t e ) .
In practical applications where e c o n o m i c considera- tions are decisive, one must also take into a c c o u n t the time and cost of studying the linguistic properties in a sufficiently large and r e p r e s e n t a t i v e s a m p l e of the s u b l a n g u a g e and of c r e a t i n g a n d p r o g r a m m i n g the s u b l a n g u a g e - s p e c i f i c d i c t i o n a r y and g r a m m a r rules. T h e r e is r e a s o n to believe that sublanguages that are s e m a n t i c a l l y and p r a g m a t i c a l l y n e a r - n e i g h b o r s are similar in their g r a m m a t i c a l properties, so that a b e t t e r u n d e r s t a n d i n g of language f o r m and f u n c t i o n will m a k e the description of new sublanguages easier and m o r e predictable.
5.5 M a t u r i n g Areas of Research R e l e v a n t to t h e S u b l a n g u a g e A p p r o a c h
A successful general a p p r o a c h to sublanguage proc- essing in a wide v a r i e t y of d o m a i n s will d e p e n d on a d v a n c e s in a n u m b e r of research areas, some of which are maturing rapidly. Empirical w o r k on k n o w l e d g e structures ( B o b r o w et al. 1977, M a r k 1980, R o b i n s o n et al. 1980) and on m e c h a n i s m s of focus ( G r o s z 1977, 1981) is relevant to a p r o p e r t r e a t m e n t of sublanguage specific features of discourse and semantic structure.
Techniques of using precise selectional restrictions f o r s u b l a n g u a g e s h a v e b e e n i m p l e m e n t e d ( B u r t o n 1976) as have those for extracting f o r m a t t e d i n f o r m a - tion f r o m fairly s t e r e o t y p e d s u b l a n g u a g e s (Sager 1978). A new technique for developing t r a n s p o r t a b l e s y s t e m s for n a t u r a l - l a n g u a g e i n t e r f a c e s to d a t a b a s e s ( H e n d r i x and Lewis 1981) elicits f r o m the user a lan- guage for querying the c o n t e n t s at the same time that i n f o r m a t i o n a b o u t the domain is being entered. This a p p r o a c h is being e x t e n d e d to provide a m o r e sophisti- cated s y s t e m that is not limited to f o r m a t t e d d a t a b a s e s but entails translation into a set of w e l l - f o r m e d f o r m u - las in a m a n y - s o r t e d f i r s t - o r d e r logic ( H a a s and H e n - drix 1980). R e c e n t w o r k on treating d e p a r t u r e s f r o m g r a m m a t i c a l i t y ( S o n d h e i m e r and W e i s c h e d e l 1980, H a y e s and M o u r a d i a n 1980, M c K e o w n 1980, K w a s n y and S o n d h e i m e r 1981, Miller et al. 1981) can be use in handling specialized language that deviates syntacti- cally f r o m the s t a n d a r d language. Devices for design- ing m o r e " f r i e n d l y " s y s t e m s , such as the w o r k on graceful interaction ( K a p l a n 1978, H a y e s and R e d d y 1979, Weischedel and S o n d h e i m e r 1981) are relevant to the question of relating sublanguage-specific p h e - n o m e n a to those of the whole language.
Richard Kittredge Sublanguages
5.6 P r o m i s i n g N e w R e s e a r c h A r e a s
A n u m b e r of new or e v e n u n d e r d e v e l o p e d research areas will certainly p r o v e i m p o r t a n t for w o r k on sub- language. We expect that further research on s y n t a c - tic variation will yield a m o r e unified f r a m e w o r k for the description of sublanguage word and phrase struc- ture. W o r k in pragmatics, such as the recent c o m p u t a - tional modeling of speech acts, will intersect with in- v e s t i g a t i o n s into s u b l a n g u a g e s w h e r e social or legal dimensions are important. As we a c c u m u l a t e experi- ence in s e m a n t i c processing o v e r a n u m b e r of specialty areas, we will be able to identify m o r e a n d m o r e sharply the i m p o r t a n t p a r a m e t e r s f o r assessing the c o m p u t a t i o n a l t r a c t a b i l i t y of a n y given s u b l a n g u a g e . This experience will also nourish a distinct area which has b o t h theoretical and practical aspects: the p r o b - lem of relating sublanguages ( a n d their g r a m m a r s ) to the standard language (and its g r a m m a r ) . T h e pre- liminary efforts at building up a t a x o n o m y and t y p o l o - gy of sublanguages are aimed in this direction.
T h e r e is already an identifiable m o v e m e n t t o w a r d s codifying and teaching language f o r specific purposes. F o r some applications it is possible to take naturally occurring sublanguages and slightly regularize t h e m so that strong tendencies are p r o m o t e d to n o r m s for c o m - municating in the subfield. A t t e m p t s in this direction have o c c u r r e d in the stylistic guidelines n o w used for writing w e a t h e r r e p o r t s and aircraft m a i n t e n a n c e m a n - uals. A serious scientific a p p r o a c h to this " e n g i - n e e r i n g d e s i g n " of n e w s u b l a n g u a g e s m u s t await a m o r e exact theoretical and practical u n d e r s t a n d i n g of h o w language function relates to language form.
5.7 R e c o m m e n d a t i o n s
At present, only a small n u m b e r of sublanguages haw~ b e e n studied in detail. Thus o n e urgent need is to b r o a d e n the basis of o u r u n d e r s t a n d i n g of these linguistic subsystems. T h e m e m b e r s of the panel feel tlhat this can best be achieved b y selecting a f e w p r o m - i,dng a p p l i c a t i o n areas in which to c o n c e n t r a t e s u b - s t a n t i v e r e s e a r c h resources. Such c o n c e n t r a t i o n is n e c e s s a r y f o r several reasons. First, m o s t naturally o c c u r r i n g s u b l a n g u a g e s p r e s e n t real challenges f o r linguistic description. M a n y m o n t h s or years of e f f o r t m u s t usually be i n v e s t e d in d e s c r i b i n g a c o r p u s of texts and in finding the natural extensions of t h a t cor- pus in c o l l a b o r a t i o n with s p e a k e r s of the sublanguage. Second, the linguistic peculiarities of the s u b l a n g u a g e o f t e n p r e s e n t new p r o b l e m s f o r c o m p u t a t i o n a l t r e a t - m e n t , particularly if the solutions are to be generaliza- ble to other, related sublanguages. Third, m a n y fur- ther m o n t h s of on-site testing are usually n e c e s s a r y to p r o p e r l y a b s o r b and e v a l u a t e the f e e d b a c k f r o m users of p r o t o t y p e s y s t e m s , and to e v o l v e m o r e a d e q u a t e versions. T h e evolution of a n y significant new s y s t e m t h e r e f o r e implies a substantial collaborative e f f o r t o v e r a period ranging f r o m several m o n t h s to several years.
6. A c k n o w l e d g e m e n t s
T h i s w o r k s h o p is t h e f i r s t in a s e r i e s o r g a n i z e d b y t h e N a v y C e n t e r f o r A p p l i e d R e s e a r c h in A r t i f i c i a l I n t e l l i g e n c e . T h e c o n c e p t f o r this w o r k s h o p e m e r g e d f r o m n u m e r o u s d i s c u s s i o n s w i t h M a r v i n D e n i c o f f a n d J o e l T r i r n b l e o f O N R , P a u l C h a p i n a n d H e n r y H a m - b u r g e r o f NSF, R o b e r t E n g e l m o r e a n d R o b e r t K a h n o f D A R P A , S t a n l e y W i l s o n a n d J o h n D a v i s o f NRL, a n d W i l l i a m P r i c e o f A F O S R .
T h e w o r k s h o p i t s e l f w a s m a d e p o s s i b l e o n l y t h r o u g h t h e s u p e r b c o o p e r a t i o n o f t h e A C L . N o r m S o n d h e i m e r , f o r m e r A C L p r e s i d e n t , a n d D o n W a l k e r , A C L S e c r e t a r y - T r e a s u r e r , u s e d t h e i r o r g a n i z a t i o n a l t a l e n t s to i n c o r p o r a t e t h e w o r k s h o p i n t o t h e 1981 A C L C o n f e r e n c e . J e r r y K a p l a n , l o c a l c h a i r m a n f o r t h e A C L m e e t i n g , g r a c i o u s l y a c c e p t e d t h e a d d e d r e s p o n s i b i l i t y o f p r o v i d i n g l o c a l a r r a n g e m e n t s f o r t h e w o r k s h o p .
W e g r a t e f u l l y a c k n o w l e d g e t h e v e r y c o m p e t e n t s e c r e t a r i a l a s s i s t a n c e b y J a n e t L. S t r o u p o f N R L a n d t h e c a r e f u l c o m p i l a t i o n o f t h e w o r k s h o p p r o c e e d i n g s b y V e r o n i c a B a t e s o f NRL. F i n a n c i a l s u p p o r t f o r t h e w o r k s h o p w a s p r o v i d e d b y t h e O f f i c e o f N a v a l R e - s e a r c h .
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