The Design and Application of a Domain Specific Knowledgebase in the TACITUS Text Understanding System

The Design and Application

of a Domain Specific Knowledgebase

in the TACITUS

Text Understanding System

Abstract

TACITUS is a text understanding system being developed at SRI In­ ternational. One o f the main components in the system is a knowledge­ base which contains commonsense and domain specific world knowledge encoded as axioms in a first order predicate calculus language. The prime function o f the knowledgebase is to provide extra-linguistic facts to be used in the resolution o f a range o f ambiguities such as compound nom­ inal constructions, definite reference, and in drawing conclusions on the basis o f the implicatures in the text. The paper discusses the methodology used in building a knowledgebase for analyzing news reports about terror­ ist attacks, and demonstrates how it is used in an application extracting information to be stored in a simulated database.

1

Preamble

D u r in g m y te r m as In te r n a tio n a l F e llo w a t S R I In te rn a tio n a l, C a lifo r n ia , th is p a s t w in te r, I h a d th e o p p o r t u n it y t o fa m ilia riz e m y s e lf w ith th e T A C I T U S t e x t u n d e r s ta n d in g s y s te m . U n d e r th e s u p e r v is io n o f J erry H o b b s , w h o is h ead o f th e T A C I T U S p r o je c t , I d e v e lo p e d a d o m a in s p e c ific k n o w le d g e b a s e f o r th e T A C I T U S s y s te m . T h e p re se n t p a p e r is a b r ie f a n d fa irly h ig h -le v e l a n d n o n ­ te c h n ic a l o v e r v ie w o f th e e n te rp rise .

S e c tio n 2 o f th e p a p e r p re se n ts th e m e t h o d o lo g y u sed in th e c o n s tr u c tio n o f th e k n o w le d g e b a s e fo r n ew s r e p o r ts a b o u t te r r o r is t a tta ck s ; a c r u d e o u tlin e o f th e T A C I T U S s y s te m is g iv e n in s e c tio n 3 as n e ce ssa ry b a c k g r o u n d in fo rm a tio n b e fo r e w e g o o n t o lo o k in g in d e ta il a t a n e x a m p le te x t in se ctio n s 4 a n d 5. W e c o n c lu d e w ith s o m e fin a l re m a rk s in s e c tio n 6.

2

The Methodology behind the Construction

of the Knowledgebase

O u r g o a l w as t o b u ild a fa ir ly la rg e k n o w le d g e b a s e fo r a s p e c ific d o m a in , n a m e ly terrorist a tta ck s , t o b e u sed as a b a sis fo r a u to m a t e d u n d e r s ta n d in g o f te x ts fa llin g w ith in th is d o m a in , a n d su b se q u e n t a u to m a t ic e x t r a c t io n o f s p e c ific in­ fo r m a tio n . W e d e c id e d t o w o r k o n th e b a sis o f a set o f s a m p le te x ts , a n d w e c o m p ile d a c o r p u s c o n s is tin g o f s ev era l n ew s r e p o r ts a b o u t te rro ris t e v en ts. T h is c o r p u s th en c o n s tit u t e d th e b a c k b o n e in o u r w ork .

R a th e r th a n a d o p t w h a t m ig h t b e te r m e d a s tr ic t s u b la n g u a g e a p p r o a c h t o th e d e s c r ip tiv e ta sk (c f. H irsch m a n 1986, a n d H o b b s 1 984 f o r m o r e d e ta ile d d is ­ cu s s io n s ), w e e m p lo y e d a m e t h o d o lo g y o f ste p w ise refin em en t (c f. H o b b s 1 9 8 4 ). T h e th ree s te p s o f o u r w o r k in g m e t h o d o lo g y , w h ich w ill b e e la b o r a t e d o n b e lo w , c o n s is te d in:

• A n (in fo r m a l) a n a ly sis o f th e c o r p u s te x ts in o r d e r t o esta b lish a b a s ic v o c a b u la r y , d e te rm in e a n d s e le ct relev a n t fa c ts fo r th e d o m a in .

• B re a k in g u p th e d o m a in in to s e lf-c o n ta in e d a n d co h e r e n t s u b -d o m a in s .

• A x io m a tiz in g th e fa c ts o f th e s u b d o m a in s .

2.1

T h e A nalysis o f C orpus T exts

F irstly , th e c o r p u s te x ts serv ed th e p u r p o s e o f e s ta b lis h in g th e b a s ic v o c a b u la r y in o u r sy stem . S e co n d ly , th e y c o n s tit u t e d a p ic t u r e o f th e w o r ld w e in te n d e d t o m o d e l in o u r k n o w le d g e b a s e , i.e. w h a t a re th e s e ttin g s , w h a t a re th e ty p ic a l a c tio n s , w h o a re th e a g en ts, w h a t a re th e ro le s a n d r e la tio n s b e tw e e n th e en titie s in o u r ‘ te r r o r is t’ u n iverse, e tc . T h u s th e y in d ic a te d w h a t lin g u is tic a n d e x tr a - lin g u istic in fo r m a tio n w o u ld b e n e e d e d in o u r k n o w le d g e b a s e .

U sin g a fu ll-s e n te n ce c o n c o r d a n c e o f th e s a m p le te x ts , w e lo o k e d a t ea ch sin gle le x ic a l ite m in c o n t e x t , a n d n o te d d o w n , in a n in fo r m a l m a n n e r, w h a t fa cts w ere lin g u is tic a lly p r e s u p p o s e d a n d w h a t g e n era l b a c k g r o u n d k n o w le d g e w o u ld b e n e e d e d in o r d e r t o u n d e rs ta n d a g iv e n o c c u r r e n c e o f a le x ic a l ite m in its c o n te x t. (W e w ill n o t d iscu ss th e m e a n in g o f ‘ u n d e r s t a n d ’ h ere, b u t w e u se it in a sen se sim ila r t o th a t o f E c o ’s te r m ‘ a c tu a lis a tio n ’ ( E c o 1 9 7 9 )).

T h e an a ly sis resu lts in a first b r e a k in g d o w n o f e a c h ite m in to c o m p o n e n t p a rts a n d e x p lic it s ta te m e n ts a b o u t th e im p lic a tu r e s (G r ic e 1 9 7 5 ) c a rrie d b y th e te x t.

2.2

Structuring th e D om ain Inform ation

S o r tin g fa c t s in to s u b -d o m a in s is g e n e r a lly a s tra ig h tfo rw a rd p ro ce s s . T h e first c r u d e d is t in c tio n w h ich ca n b e m a d e , is th a t b e tw e e n fa c ts p e rta in in g to c o m m o n s e n s e k n o w le d g e a n d d o m a in s p e c ific o r sp e cia liz e d k n ow led g e. T h e fo r ­ m e r is fa c t s a b o u t th e w o r ld in g e n era l a n d n o t p a r tic u la r ly tied t o a sp e c ific d o ­ m a in ( b e it te r r o r is t a c tio n s , in fo r m a tio n te ch n o lo g y , o r w h a t h a v e y o u ), w h ereas th e la t te r ch a r a c te r iz e s th e fa c t s w h ich a re q u ite o fte n fo u n d t o b e re stricte d a n d h ig h ly s p e c ia liz e d .

F a cts p e r ta in in g t o fo r e x a m p le s p a c e , tim e, a n d b e lie f a re co n s id e re d c o m ­ m o n s e n s e k n o w le d g e , w h e re a s v a rio u s fa c t s a b o u t te r r o r is t o rg a n iz a tio n s are c le a r ly d o m a in s p e c ific , a n d essen tia l fo r th e u n d e rs ta n d in g o f r e p o r ts a b o u t te r r o r is t e v en ts. G e o g r a p h ic a l fa c t s a b o u t th e lo c a t io n o f citie s a n d cou n tries se e m t o fa ll s o m e w h e r e b e tw e e n th e m o r e a b s tr a c t c o m m o n s e n s e n o tio n a n d th e s p e c ia liz e d d o m a in k n o w le d g e .

O n th e b a sis o f th e resu lts fr o m o u r fa c t-fin d in g , i.e. s te p o n e a b o v e , w e d e fin e d 3 0 s u b -d o m a in s . T h e o v e ra ll c o n c e p t u a l s tru ctu re fo r th e k n o w le d g e b a se, th e s u b -d o m a in s a n d th e r e la tio n s b e tw e e n th e m , ca n b e s c h e m a tic a lly ren dered b y th e illu s tr a tio n in fig u re 1.

A p a r t fr o m p r o v id in g c o n c e p t u a l cla rity , th e a d v a n ta g e o f th is m o d u la r a p ­ p r o a c h is o b v io u s ly th a t it p e r m its y o u t o la ter en h a n ce o r m o d ify th e su b - d o m a in s in th e k n o w le d g e b a s e in d e p e n d e n tly o f ea ch o th e r.

2 .3

A x io m a tiza tio n o f th e Facts

T h e fin a l s te p in th e c o n s tr u c tio n o f th e k n o w le d g e b a s e c o n s is te d in cre a tin g p r e c is e o n t o lo g ie s fo r th e in d iv id u a l s u b -d o m a in s , i.e. w h a t en tities e x is t an d w h a t a re th e re la tio n s b e tw e e n th e m , a n d a x io m a tiz in g th e fa cts.

T h e m a in ta sk h ere w a s t o d e c id e o n w h ich p r e d ic a te s t o d e c o m p o s e , i.e. c h a r a c te r iz e b y o t h e r o r n ew p r e d ic a te s , a n d w h ich w e re t o b e b a s ic p re d ica te s, i.e. g r o u n d te rm s fo r w h ich n o fu rth e r d e s c r ip t io n is p r o v id e d .

T h e id e a b e h in d th e a d o p t e d a p p r o a c h is n e ith e r t o fu lly d efin e ea ch lex ica l ite m in th e sen se o f p r o v id in g n e ce ssa ry a n d su fficien t c o n d itio n s , n o r t o d e c o m ­ p o s e it in to a p re d e fin e d set p rim itiv e s in th e S ch a n k ia n tr a d itio n . R a th e r , th e p u r p o s e w a s t o c h a r a c te r iz e th e p r e d ic a te s u sed in th e k n o w le d g e -b a s e . C o n s id e r as a n e x a m p le th e fo llo w in g a x io m s fr o m th e ‘o r g a n iz a tio n ’ s u b -d o m a in .

o r g a n iz a tio n ( o ) - > E s ( V x . x G s - > p e r s o n ( x ) & m e m b e r ( x , o ) ) & E p ,g p la n ( p ,g ,o )

m e m b e r ( x ,o ) - > E e. r o le ( e ,x ,o )

r o le ( e ,x ,o ) < - a g e n t (x ,e ) & in .s e r v ic e jD f ( e ,g ,p ) & p la n ( p ,g ,o )

bomb.attack kidnap.attack

I

terrorist.organization

I

I

I

country

I

ethnicity

I— organization —I

1

police

I

-I

city

-I

publish—

I---firm

plansftgoeds

--- communication

I

property

I

I

daily_life

I

I

belief

1

possession

person.move

injury

----I

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process

I

1

time

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numbers |

-bomb_structure

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— phys_obj

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I-normal

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modality

I-■— predicates

h a s a r o le , w h ich is b e in g th e a g en t o f s o m e a c tio n w h ich is in se rv ice o f th e plan o f th e o r g a n iz a tio n .

3

The Knowledgebase and the TACITU S

System

T o te st o u r k n o w le d g e b a s e , w e im p le m e n te d a su b set (a p p . 1 0 0 ) o f th e a x io m s w e h a d d e fin e d o n th e s y s te m , a n d ran d ifferen t ty p e s o f se n te n ce s. T h e a x io m s are s ta te d in th e ‘ o n t o lo g ic a l p r o m is c u o u s ’ n o ta tio n d e v e lo p e d b y H o b b s (c f. H o b b s 1 9 8 5 b ).

T h is n o ta t io n is a first o r d e r p r e d ic a te ca lc u lu s la n g u a g e w ith th e a d d itio n o f a n o m in a liz a tio n o p e r a t o r , w r itte n ‘ !’ , a n d an e x tr a a rg u m e n t, in fo r m a lly referred t o as th e ‘ s e lf ’ a rg u m e n t.

T o b e m o r e c o n c r e t e a n d t o c o n v e y th e b a s ic in tu ition o f th e n o ta tio n t o th e r e a d e r, le t us c o n s id e r a s im p le e x a m p le :

e x p lo d e ( b ) w h ich is t o b e rea d as: b e x p lo d e s

e x p l o d e ! ( e l ’", b ) w h ich is t o b e rea d as: th e e x p lo s io n o f b.

W h e r e p ( x ) sa y s th a t p is tru e o f x , p !( e ,x ) says th a t e is th e e v e n tu a lity o r p o s s ib le s itu a tio n o f p b e in g tru e o f x . C o n se q u e n tly , H o b b s ’ n o ta tio n c a n b e re la te d t o s ta n d a r d first o r d e r p r e d ic a te ex p ression s b y th e fo llo w in g a x io m :

( V x ) p ( x ) < = > (E e ) p !( e ,x ) & R e x is ts (e )

w h e re R e x is t s ( e ) sa y s th a t th e e v e n tu a lity ‘e ’ d o e s in fa c t re a lly ex ist.

In s u m , th e b a s ic id e a o f th e n o ta tio n is th a t o f s p littin g a sen te n ce in to its p r o p o s it io n a l c o n te n t a n d a n a s s e r tio n a l/e x is te n tia l c la im . F u rth e rm o re , th e self a r g u m e n t, i.e. th e ‘ e ’ , p r o v id e s a ‘ h a n d le ’ fo r referrin g t o a p r e d ic a tio n , i.e. a p r e d ic a te a n d its a r g u m e n t, in o th e r p r e d ic a te s .

B e fo r e w e g o o n t o d is cu s s in g a s a m p le t e x t, w e w ill g iv e a c ru d e o v e rv ie w o f th e b a s ic c o m p o n e n t s a n d th e ir fu n c tio n in g in th e T A C I T U S sy ste m . W e d e lib e r a te ly ig n o r e s o m e o f th e m o r e a d v a n c e d fea tu res o f T A C I T U S in o r d e r n o t t o g e t b o g g e d d o w n b y t o o m a n y te c h n ic a l d eta ils. U n fo rtu n a te ly , th is m ea n s th a t w e d o n o t d o T A C I T U S fu ll ju s t ic e (b u t f o r m o r e d e ta ile d a n d co m p re h e n s iv e d e s c r ip t io n s o f th e s y s te m , see fo r e x a m p le H o b b s 1 9 8 6 c a n d la te r).

th e p ra g m a tics m o d u le is t o re so lv e referen tia l e x p r e s s io n s a n d s o m e s y n t a c t ic a m b ig u ities, t o e x p a n d m e to n y m ie s , a n d t o in te rp re t th e im p lic it r e la tio n s in c o m p o u n d n om in a ls. T h e p r a g m a tic s m o d u le w o rk s b y c o n s t r u c t in g a lo g ic a l ex p ression fo r th e b a s ic se m a n tic a n a ly sis resu lt, a n d c a llin g th e K A D S th e o r e m p ro v e r (S tick el 1 9 8 2 ) t o p r o v e o r d e riv e it u sin g a s ch e m e o f a b d u c t iv e in fe r e n c in g in w h ich it is p e r m itte d t o a ssu m e th e e x is te n c e o f ‘ n e w ’ fa c ts . T h e th e o r e m p ro v e r d ra w s o n th e k n o w le d g e b a s e o f c o m m o n s e n s e a n d d o m a in k n o w le d g e t o c o m p le te th e ta sk .

A b d u c t iv e in fe re n ce is, o f c o u r s e , a lo g ic a lly in v a lid m o d e o f in fe re n ce , i.e. g iv en p ( X ) —► q ( X ) a n d q ( a ) w e c o n c lu d e p ( a ) . H o w e v e r, w e m a y a rg u e , as d o e s H o b b s (c f. H o b b s e t al. 1 9 8 8 ), th a t it is a r e a so n a b le w a y o f lo o k in g a t te x t u n d e rsta n d in g b e c a u s e a b d u c t io n is in fe re n ce t o th e ‘ b e s t e x p la n a t io n ’ in a g iv e n c o n te x t. q ( a ) c a n b e th o u g h t o f as th e o b s e r v e r a b le e v id e n c e , th e im p lic a t io n as th e gen era l p r in c ip le th a t c o u ld e x p la in th e o c c u r e n c e o f q ( a ) , a n d th e a n te c e d e n t o f th e im p lic a t io n as th e u n d e rly in g ca u s e o r e x p la n a tio n o f q ( a ) .

A n in terestin g fe a tu re o f th e p r a g m a tic s m o d u le is th a t it u ses a s ch e m e fo r a b d u c tiv e in fe re n cin g in w h ich w eig h ts a n d c o s ts a re a ssig n e d t o th e a x io m s (fo r fu rth er d eta ils, see e .g . S tick el 1 9 8 8 ). T h u s i f w e c a n n o t p r o v e an a n te c e d e n t, w e assu m e its e x is te n c e a t s o m e c o s t. S o m e b a s ic h e u r is tic p r in c ip le s c o n tr o llin g th e w eig h ts a n d a ssu m a b ility c o s ts a re h a rd w ire d in to th e s y s te m (e .g . it is m o r e e x p e n siv e t o a ssu m e a fa c t th a n t o p r o v e it, a n d it is less e x p e n s iv e t o a ssu m e an in d efin ite e n tity th a n a d e fin ite o n e ), b u t th e a x io m s in th e k n o w le d g e b a s e m a y b e a ssign ed co s ts m a n u a lly (c f. 4 .2 ). T h e in te r p r e ta tio n o f a t e x t in th is a b d u c tiv e a n d a s s u m p tio n -b a s e d fr a m e w o r k , a m o u n ts t o p r o d u c in g th e m in im a l e x p la n a tio n o f w h y th e te x t w o u ld b e tr u e (c f. H o b b s et al. 1988 fo r a d e ta ile d d iscu ssio n ).

T h e an a ly sis c o m p o n e n t, i.e th e c o m p o n e n t fo r e x tr a c tin g ta sk s p e c ific in­ fo r m a tio n fr o m a n in te rp re te d t e x t , is b a s ic a lly a s p e c ia liz e d c a ll t o th e th e o r e m p ro v e r (see fu rth e r b e lo w ). T h e e n h a n c e d lo g ic a l fo r m , i.e. th e resu lt o u t p u t fr o m th e p r a g m a tic s m o d u le , is a b d u c t iv e ly p r o v e d b y b a c k -c h a in in g o v e r th e a x io m s in th e k n o w le d g e b a se .

In th e n e x t s e c tio n s , w e w ill h a v e a lo o k a t a n e x a m p le te x t a n d s h o w h o w th e k n o w le d g e b a s e is u sed fo r d is a m b ig u a tio n a n d c o m p u t a t io n o f im p lic it in fo r ­ m a tion .

4

An Example

L et us n o w co n s id e r th e fo llo w in g tw o s en ten ces as an e x a m p le te x t t o b e tr e a te d w ith in o u r fra m e w o rk :

(1 ) A b o m b e x p lo d e d a t a R e n a u lt s h o w r o o m in B ilb a o . A p e r s o n c la im in g t o represen t th e E T A - M h a d w a rn e d o f th e b la st in a c a ll t o th e p o lic e .

T h e e x tr a -lin g u is tic k n o w le d g e n e e d e d in o r d e r t o a ch ie v e s o m e rea son a b le le v e l o f u n d e r s ta n d in g o f th e te x t is a m o n g o th e r th in g s; R e n a u lt is a F rench firm m a n u fa c tu r in g p r o d u c t s , i.e. ca rs , a s h o w r o o m is a b u ild in g o w n e d b y a firm w h e r e th e p r o d u c t s o f th a t firm a re o n d isp la y, B ilb a o is a c it y in th e c o u n t r y S p a in , E T A - M is a te r r o r is t o r g a n iz a tio n , a n d te rro rist org a n iz a tio n s h a v e m e m b e r s , ce rta in p la n s a n d g o a ls a n d v io le n t m e t h o d s fo r rea ch in g th eir g o a ls , a n d a n e x p lo s io n g e n e r a lly in v o lv e s a blast.

T h e b a s ic fa c t s su ch as fo r in s ta n c e S p a in b e in g a c o u n t r y a n d E T A -M b e in g a te r r o r is t o r g a n iz a tio n , a re e n c o d e d as e x isten tia l a x io m s in th e k n ow led g eb a se. E .g :

(la) (Defaxiom COUNTRY-SPAIN-1 (terror)

‘‘

Spain is a country’’

((SOME ((el* . ev) (country! el* Spain)))

(lb) (Defaxiom TERORG-ETA-M-1 (terror)

“ETA-M is a terrorist orgemization’

’

((SOME ((el* . ev) (terorgi el* eta-m)))

T h e q u a n tifie d v a ria b les in th e a x io m s a re m a rk ed fo r th eir t y p e su ch th a t ‘e v ’ d e n o te s ev e n t a n d ‘ n e v ’ n o n -e v e n t v a ria b les.

4 .1

A x io m s for D isam biguating C om pou n d Nomined

Constructions

F r o m th e lin g u is tic p o in t o f v ie w , th e T A C I T U S fr a m e w o r k offers in terestin g p o s s ib ilitie s fo r d is a m b ig u a tin g c o m p o u n d n o m in a l e x p r e s s io n s u sin g lin g u istic as w ell as e x tr a -lin g u is tic k n o w le d g e .

T h e in d iv id u a l n o u n s in a c o m p o u n d n o m in a l c o n s tr u c tio n a re a n a ly z e d as a rg u m e n ts o f th e g e n e r ic ‘ n n ’-p r e d ic a t e . T h a t is, th e e x p re s s io n ‘ R e n a u lt sh ow ­ r o o m ’ , w o u ld a p p e a r as n n ( e l ’'‘ , R e n a u l t , S h o w r o o m ) in th e in itia l lo g ic a l fo rm o f th e s e n te n ce p r o d u c e d as o u t p u t fr o m th e p a rsin g m o d u le .

In fo r m u la tin g th e a x io m s fo r re s o lv in g su ch n n -re la tio n s , w e a d o p te d a stra t­ e g y c o m b in in g th e lin e o f a n a ly sis fo r c o u m p o u n d n o m in a ls p r o p o s e d b y D o w n ­ in g (1 9 7 7 ), a n d th a t a d v o c a t e d b y L e v i (1 9 7 8 ). In su m m a ry . D o w n in g argu es th a t th e se m a n tic re la tio n s h ip b e tw e e n th e elem en ts o f a c o u m p o u n d c a n n o t b e c h a r a c te r iz e d in te r m s o f a fin ite list o f a p p r o p r ia te c o m p o u n d in g rela tion sh ip s, w h e re a s L e v i tries t o e s ta b lis h su ch a list fo r th e m o s t c o m m o n ca ses o n th e b a sis o f th e tr a n s fo r m a tio n a l re la tio n s h ip b etw e e n th e elem en ts.

(2a) (Defaziom NN-1 (terror)

“An nn-relation: for”

(ALL ((el* . ev) (p . nev) (s . nev))

(IMPLY (for! el* s p)

(SOME ((e2* . ev))

(nn! e2* p s)))))

(2b) (Defaziom NN-2 (terror)

“An nn-relation: of”

(ALL ((el* . ev) (f . nev) (s . nev))

(IMPLY (of! el* s f)

(SOME ((e2* . ev))

(nn! e2* f s)))))

(3a) (Defaziom FOR-1 (terror)

“A shovroom is for products’’

(ALL ((e2* . ev) (s . nev) (e3* . ev) (p . nev) (e4* . ev) (f . nev))

(IMPLY (AHD (shovroom! e2* s) (product! e3* p) (firm! e4* f))

(SOME ((el* . ev))

(for! el* s p)))))

(3b) (Defaziom OF-1 (terror)

“A shovroom is ovned by a firm’’

(ALL ((o2* . ev) (s . nev) (e3*. ev) (e4* . ev) (f . nev))

(IMPLY (AND (shovroom! e2* s) (ovn! e3* f s) (firm! e4* f))

(SOME ((el* . ev))

(of! el* s f)))))

In tr y in g t o a b d u c tiv e ly p r o v e a relev a n t lo g ic a l fo r m o u t p u t fr o m th e p a rsin g m o d u le a n d t o m a k e im p lic it in fo r m a tio n e x p lic it , th e p r a g m a tic s m o d u le haa th e th e o r e m p ro v e r b a c k -c h a in o v e r th e a x io m s in th e k n o w le d g e b a s e . T h u s an n n -re la tio n as th e a b o v e is r e so lv e d a g a in st 2 a a n d 2 b , th e n th e n ew g o a ls , o f ! ( e l * s f ) a n d f o r ! ( e l * s f ) , a re r e s o lv e d a g a in st 3 a a n d 3 b re s p e ctiv e ly , y ie ld in g n ew g o a ls t o b e re solv ed .

4.2

A x io m s for R esolving R eferring Expressions

A s m e n tio n e d a b o v e , o n e o f th e b a s ic h e u ristic a s s u m p tio n h a rd w ire d in to T A ­ C I T U S ’ p r a g m a tic s m o d u le is th a t an in d e fin ite n o u n p h ra se in tr o d u c e s n ew in fo rm a tio n a n d a d e fin ite n o u n p h ra se refers t o a k n o w n en tity , i.e. s o m e th in g w h ich is eith er in th e k n o w le d g e b a s e o r h a s b e e n in tr o d u c e d in th e p r e v io u s ly p r o c e s s e d te x t. H e n ce th e c o s t o f a ssu m in g an in d e fin ite n o u n p h ra se is c h e a p e r th a n a ssu m in g a d e fin ite n o u n p h rase.

In th e e x a m p le s en ten ces g iv e n in (1 ) , th e n o u n p h ra se ‘ th e b la s t ’ , is rela ted t o th e ev en t o f th e e x p lo s io n m e n tio n e d in th e p r e c e e d in g se n te n ce . S im p lify ­ in g so m e w h a t (c f. fu rth e r b e lo w ), w e c o u ld sa y th a t ’ th e b la s t ’ is in a sen se a n o m in a liz a tio n o f ‘ a b o m b e x p lo d e d ’ .

(4) (Defaxiom EXPLOSIOK-BLAST-1 (terror)

‘‘

An explosion generates a blast'’

(ALL ((el* . ev) (x . nev) (y . nev) (z . nev))

(IMPLY (AND (ASSUMABLE (etc-expl el* x y z ) 0.3)

(explode! el* x y z))

(SOME ((e2* . ev) (b . nev))

(AND (blast! e2* b) (genn el* e2*))))))

E ssen tia lly , th is a x io m sa y s th a t a b la s t (e 2 * ) im p lies th e o c c u r r e n c e o f som e e x p lo s io n ev e n t ( e l * ) , a n d th a t th e la tte r g en era tes th e fo r m e r , w h ich is sta ted b y w a y o f th e p r im itiv e p r e d ic a te ‘ g e n n ’ . T h e p r e d ic a te ‘e t c - e x p l’ , w h ich ca n b e seen as ‘ a d d it io n a l’ , b u t n o t s p e lle d o u t p r o p e r tie s r e la tin g t o th e e x p lo d e p r e d ic a te , is in t r o d u c e d b e c a u s e w e d o n o t w a n t t o s ta te fla tly th a t ‘ a b la s t’ and ‘ a n e x p lo s io n ’ is th e sa m e th in g .

S in ce a n ‘ e x p lo s io n ’ is k n o w n (it w a s in tr o d u c e d in th e p r e v io u s s e n te n c e ), it is free o f c h a r g e t o re s o lv e th e s e c o n d p r e d ic a te in th e a n te ce d e n t o f th e a x io m a g a in s t th is k n o w n fa c t . T h e first p r e d ic a te in th e a n teced en t h as b e e n assign ed su ch a lo w a s s u m a b ility c o s t ( 0 .3 ), th a t p r o v in g ‘ b la s t’ b y u se o f th e a x io m is c h e a p e r th a n t o a ssu m e its e x is te n ce .

5

Extracting Specific Information from

the Texts

T h e lo g ic a l fo r m e n c a p s u la tin g th e in te r p r e ta tio n fo u n d fo r a t e x t, i.e. th e o u tp u t fr o m th e in te r p r e ta tio n c o m p o n e n t , is th e in p u t t o th e ta sk s p e c ific a n alysis c o m p o n e n t . T h e a n a ly sis is p e r fo r m e d o n th e b a sis o f th e lo g ic a l fo r m a n d a ‘ ta sk s c h e m a s p e c if ic a t io n ’ g iv e n t o th e th e o r e m p r o v e r .

5.1 T h e Schem a

L e t us h ere c o n s id e r a sim p lifie d e x a m p le o f th e k in d o f ev en t rela ted sp ecific in fo r m a tio n w e w o u ld lik e th e s y s te m t o c o m p u t e . F o r a g iv en te x t d e s c r ib in g a te r r o r is t e v e n t, w e w o u ld lik e t o fin d an sw ers ( i f a n y ) t o ‘ q u e s tio n s ’ su ch as th e fo llo w in g :

INCIDENT TYPE:

TARGET TYPE:

TARGET NATIONALITY:

INCIDENT CITY:

INCIDENT COUNTRY:

RESPONSIBLE ORGANIZATION:

etc.

en tries, th e ajisw ers fo u n d a re p r in te d o u t o n th e scre e n . T h e s lo t s in th e ‘ r e c o r d ’ a re filled b y th e valu es fo u n d fo r v a ria b les w h en p re s e n tin g th e th e o r e m p r o v e r w ith g oa ls t o b e a b d u c t iv e ly p r o v e n b y u sin g th e in fo r m a tio n fr o m th e te x t in terp reted a n d th e fa c ts in th e k n o w le d g e b a s e .

T h e g o a ls o f th e s c h e m a a p p e a r as th e c o n s e q u e n t in w h a t m ig h t in fo r m a lly b e ca lle d th e ‘ lin k in g a x io m s ’ in th e a p p lic a tio n ta sk s p e c ific p a rt o f th e k n o w l­ e d g eb a se. L in k in g a x io m s c a n b e th o u g h t o f as g u id e lin e s fo r h o w t o fin d an sw ers t o th e ‘ q u e s tio n s ’ p o s e d b y w a y o f th e s c h e m a s p e c ific a tio n .

T h e s c h e m a it s e lf is a m e ta lo g ic a l L IS P e x p re s s io n in a fir s t-o r d e r p r e d ic a te ca lcu lu s fo r m a n n o t a te d b y n o n -lo g ic a l o p e r a to r s fo r sea rch c o n t r o l a n d r e s o u r c e b o u n d s . T h e t w o n o n -lo g ic a l o p e r a to r s a re ‘ p r o v in g ’ a n d ‘e n u m e r a te d -fo r -a ll’ . W it h o u t g o in g in to te ch n ica l d eta ils a b o u t th ese t w o o p e r a t o r s ( f o r m o r e d e ta ils , see T y s o n a n d H o b b s 1 9 8 8 ), let us s im p ly p resen t a sm a ll e x c e r p t fr o m th e s ch em a fo r th e a b o v e e x a m p le ‘ r e c o r d ’ , a n d meike s o m e e x p la n a t o r y c o m m e n t s in o r d e r t o c o n v e y th e b a s ic in tu itio n s o f th e p r o c e s s t o th e rea d er:

(proving

(enumerated-for-all ((el . ev))

(proving

(some ((it . nev)) (incident-type el it))

(terror-limits default-time)

print-incident)

(and

(enumerated-for-all ((it . nev))

(proving

(incident-type el it)

(terror-limits default-time)

print-incident-type)

:true)

(enumerated-for-all ((ro . nev))

(proving

(responsible-organization el ro)

(terror-limits default-time)

print-responsible-organization)

:true)

(terror-limits default-time)

print-sentence-finished)))

T h e lin k in g a x io m in th e k n o w le d g e b a s e fo r ‘ re s p o n s ib le o r g a n iz a t io n ’ c o u ld b e th e fo llo w in g sta te m e n t:

(5) (Defaziom RESP-ORG-1 (terror)

'‘

The organization responsible for the attack’’

(ALL ((el* . ev) (e . ev) (e2* . ev) (o . nev) (e3* . ev))

(IMPLY (AHD (terattack! el* e) (responsible! e2* o e)

T h u s , w e fin d th e o r g a n iz a tio n ( o ) re s p o n s ib le fo r an a tta c k (e ) b y p ro v in g th a t e is a te rro ris t a tt a c k , th a t o is a te rro rist o r g a n iz a tio n , a n d th a t o is r e s p o n s ib le f o r e.

C o n t r a r y t o th e p r a g m a tic s m o d u le , n o a s su m p tio n s a re m a d e in th e task s p e c ific a n a ly sis p h a se w h e n tr y in g t o p r o v e th e g o a ls o f th e s ch em a ; th is step is m e a n t t o e x t r a c t in fo r m a tio n o n ly . H o w ev er, th e p r o c e s s is still b a ck -ch a in in g c o n tr o lle d a b d u c t iv e in fe r e n c in g . T h is m ea n s th a t e v e r y th in g has t o b e p rov ed a g a in st th e k n o w le d g e in th e d a ta b a s e in c o n ju n c tio n w ith th e in te rp re ta tio n o f th e te x t.

P r o v in g th e a n te c e d e n ts o f th e lin k in g a x io m s m a y o f c o u r s e in v o lv e resolv in g th e n e w g o a ls w ith k n o w le d g e a sse rte d in th e te x t o r in th is c a s e , p r o v in g fu rth er a x io m s in th e k n o w le d g e b a s e .

T h e r e m a y a ls o b e d ifferen t a x io m s fo r th e sa m e g o a l, in d ic a t in g th a t a g oa l c a n b e e x p la in e d , o r m o r e c o r r e c t ly p r o v e d , in differen t w a y s. A c tu a lly , this is o n ly a r e fle c tio n o f th e fa c t th a t a g iv e n p h e n o m e n a ca n b e b r o u g h t a b o u t in d ifferen t w a y s. F o r e x a m p le , th e re a re a c tu a lly th ree d ifferen t a x io m s fo r ‘ r e s p o n s ib le ’ in o u r k n o w le d g e b a s e .

5.2

T h e In form ation E x tracted from the Interpretation

R esu lt

L e t us n o w re tu rn t o o u r e x a m p le t e x t. F or illu stra tio n , w e first sh o w an e x ­ c e r p t fr o m th e resu lt o f th e in te r p r e ta tio n o f th e sen ten ces in e x te rn a l fo r m a t (6 ) — n o te th e r e so lv e d c o m p o u n d in g rela tio n sh ip ; a n d th en th e p r in t-o u t o f th e in fo r m a tio n a u to m a t ic a lly e x t r a c t e d b y th e a n a ly z e c o m p o n e n t fr o m th e in te r p r e t a tio n (7 ) o f th e t w o e x a m p le sen ten ces.

(6)

INTERPRETATION 1 OF SENTENCE:

C o s t : 34

New emd Assumed Information:

xl:

bomb!(e2, xl)

yl:

explode!(e4, yl, xl)

xl2:

bilbaol(el3, xl2)

x8:

renault!(e9, x8)

x6:

showroom!(e7, x6)

in!(ell, x6, xl2)

e4:

at!(e5, e4, x6)

past!(el5, e4)

Given or Inferred Information:

x 8 :

renault!(e9, x8)

nn!(elO, x8, x6)

own!(e25, x8, x6)

firm!(o26, x8)

(7)

INCIDENT TYPE: explosion

TARGET TYPE: commercial

TARGET NATIONALITY: french

INCIDENT CITY: bilbao

INCIDENT COUNTRY: Spain

PROPERTY DAMAGE: <unknown>

WARNING: yes

METHOD: phone

RESPONSIBLE ORGANIZATION: eta-m

6

Final Remarks

T A C I T U S o ffers an in te re s tin g fr a m e w o r k fo r e x p e r im e n tin g w ith k n o w le d g e - b a sed n a tu ra l la n g u a g e p r o c e s s in g , a n d in fa c t it is a q u ite s o p h is tic a te d s y s te m . P re v io u sly , th e T A C I T U S te a m a t S R I h as b e e n e x p e r im e n tin g w ith im p le m e n ­ ta tio n s o f k n o w le d g e b a s e s fo r d o m a in s su ch as th e b r e a k -d o w n o r m a lfu n c t io n in g o f m e ch a n ica l p a rts in sh ip s (H o b b s 1 9 8 7 ). C o n s t r u c t in g a k n o w le d g e b a s e fo r th e te rro rist a tta c k d o m a in w as th e first a t t e m p t t o d e a l w ith a s lig h tly less r e s tr ic t­ ed s u b je c t field in th e T A C I T U S s y s te m . T h e m a in c o n c lu s io n t o b e d ra w n fr o m th e e x p e rim e n t w ith th e te rro ris t te x t s is th a t v e r y ca r e fu l a x io m a tiz a tio n o f th e f

2

w;ts is n e cessa ry in o r d e r t o a ch ie v e g o o d resu lts, i.e. ‘ n u ts a n d b o lt s ’ h a v e t o b e ca re fu lly fitte d to g e th e r t o c r e a te ‘ d e lu s io n s o f g r a n d e u r ’ .

Ack nowledgements:

T h e D a n ish C a r ls b e r g F o u n d a tio n p r o v id e d th e fin a n cia l s u p p o r t fo r m y s ta y at S R I In te rn a tio n a l. C o n s t r u c t in g a n d te s tin g th e d o m a in s p e c ific k n o w le d g e b a s e fo r te rro rist te x ts in th e T A C I T U S s y s te m d e s c r ib e d h ere, w a s s u g g e s te d t o m e b y J e rry H o b b s a n d ca rrie d o u t u n d e r his s u p e r v is io n . I a m in d e b te d t o J e rry fo r his g u id a n c e a n d m a n y u sefu l h in ts. N eed less t o say, i f th e p resen t p a p e r co n ta in s e rrors o r m is c o n c e p tio n s in th e p re s e n ta tio n o f T A C I T U S , th e a u th o r a lon e c a n b e b la m e d .

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