O s t e n D a h l
U n i v o f S t O G K h o l i n
T H E liMTERPRETATlON OF B O U N D P R O N O U N S
This p a p e r is a r e p o r t o n w o r k in p r o g r e s s w i t h t h e a i m o f s i m u l a t i n g o n a c o m p u t e r s o m e a s p e c t s o f t h e p r o c e s s o f u n d e r s t a n d i n g sentences, m o r e s p e c i f i c a l l y the i n t e r p r e t a t i o n of s o - c a l l e d b o u n d pronouns. The w o r k c o n n e c t s d i r e c t l y to s o m e e a r l i e r p a p e r s of m i n e (D a h l 1 9 8 3 a a n d 19 8 3 b ) , w h e r e t h o s e p r o b l e m s w e r e d i s c u s s e d f r o m a t h e o r e t i c a l p o i n t of view.
A b o u n d p r o n o u n is, r o u g h l y s p e a k i n g , a p r o n o u n w h i c h b e h a v e s a n a l o g o u s l y t o a b o u n d v a r i a b l e in lo gi c. T h e r e a r e a t l e a s t tw o k i n d s o f c r i t e r i a f o r r e g a r d i n g a p r o n o u n as b o u n d : (i) s y n t a c t i c c r i t e r i a - s o m e k i n d s o f p r o n o u n s , s u c h a s r e fl ex iv es , r e c i p r o c a l s a n d s o - c a l l e d i o g o p h o r i c s , m u s t f i n d t h e i r a n t e c e d e n t s in s y n t a c t i c a l l y d e f i n e d d o m a i n s , (ii) s e m a n t i c c r i t e r i a - s o m e p r o n o u n s c a n n o t be a s s i g n e d r e f e r e n t s 'in t h e w o r l d ' b u t c a n be u n d e r s t o o d o n l y if r e g a r d e d as r e f e r e n t i a l l y d e p e n d e n t o n t h e i r a n t e c e d e n t s : t h i s c o n c e r n s e.g. p r o n o u n s b o u n d b y q u a n t i f i e d N P s a n d w h - p h r a s e s (e.g. h i m s e l f in N o b o d y l i k e s h i m s e 1 f ). A l t h o u g h t h e c l a s s e s o f p r o n o u n s d e l i m i t e d by t h e s e c r i t e r i a a r e n o t q u i t e i d e n t i c a l , t h e y o v e r l a p t o s u c h a n e x t e n t t h a t in m o s t c a s e s , t h e y c a n be r e g a r d e d as eq ui va le nt .
In m y e a r l i e r p a p e r s , 1 h a v e d i s c u s s e d s o m e c a s e s o f b o u n d p r o n o u n s w h i c h a r e t r o u b l e s o m e f o r t h e c u r r e n t t h e o r i e s t h a t a c c o u n t f o r b o u n d p r o n o u n s b y t r a n s l a t i n g t h e m i n t o s o m e k i n d of l o g i c a l n o t a t i o n u s i n g b o u n d v a r i a b l e or e q u i v a l e n t devices. T h o s e c a s e s include:
(i) ' s l o p p y i d e n t i t y ' c a s e s (as in J o h n l o v e s h i s w i f e a n d so does B i l l , w h e r e Bill m a y be u n d e r s t o o d to love e i t h e r h i s o w n or John's wife)
b e e n 'moved' ( p r e s u pp os in g a t r a n s f o r m a t i o n a l anal ys is ) out of the s c o p e of t h e i r b i n d e r s , e.g. H i m self, e v e r y o n e d e s p i s e s , and in p a r t i c u l a r a m o n g th os e
(iii) p r o n o u n s w i t h ' r e l a t i o n a l ' (Engdatil 1985) o r ' s e c o n d - order' r e a d i n g s , as in a s e n t e n c e s u c h as T h ^ o n l y womari e v e r y E n g l i s h m a n a d m i r e s is h i s m o t h e r , t h e i n t e r p r e t a t i o n of w h i c h c a n n o t be r e n d e r e d w i t h o u t h a v i n g r e c o u r s e to s e c o n d - o r d e r logic
T h e m a i n idea p u t f o r w a r d in m y p a p e r s w a s th at the t r o u b l e s o m e c a s e s c o u l d be a c c o u n t e d for if the l o c a t i o n of the a n t e c e d e n t in the s y n t a c t i c s t r u c t u r e w e r e c o n s i d e r e d an i n t e g r a l p a r t of a b o u n d p r on ou n' s in te r p r e t a t i o n .
-50-' t r o u b l e s o m e c a s e s -50-' l i s t e d a b o v e . V e r s i o n 1 w a s t h u s a b l e t o h a n d l e b o t h s l o p p y i d e n t i t y a n d a t l e a s t s o m e ' r e l a t i o n a l qu es ti on s' , e.g. t h e f o l l o w i n g :
(1) W h o m d o e s e v e r y m a n love, h i s w i f e o r M a r y ?
However, V e r s i o n 1 w a s r a t h e r s l o w , w i t h p r o c e s s i n g t i m e s u p t o h a l f a m i n u t e for p r o c e s s i n g s o m e s e n t e n c e s (this w o u l d i n cl ud e b o t h s y n t a c t i c parsing, r e f e r e n c e a s s i g n m e n t , c o m p a r i s o n w i t h the d a t a b a s e a n d a p p r o p r i a t e r e a c t i o n ) . T h e r e w e r e s e v e r a l r e a s o n s f o r t h a t , i n c l u d i n g i n h e r e n t l i m i t a t i o n s in t h e h a r d w a r e a n d s o f t w a r e u s e d . O f a m o r e d i r e c t l i n g u i s t i c relevance, h o w e v e r , w e r e t h e f o l l o w i n g c i r c u m s t a n c e s : T h e s y n t a c t i c a n d s e m a n t i c c o m p o n e n t s o f t h e s y s t e m s w e r e w h o l l y a u t o n o m o u s f r o m e a c h o t h e r , a n d i n d e e d w o r k e d in r a t h e r d i f f e r e n t fashions: the s y n t a c t i c p a r s e r w a s s t r i c t l y b o t t o m - up, s y s t e m a t i c a l l y t a k i n g i n t o c o n s i d e r a t i o n s a l l p o s s i b l e a n a l y s e s o f t h e s e n t e n c e s , w h e r e a s t h e s e m a n t i c s , as h a s a l r e a d y b e e n p o i n t e d out, w o r k e d f r o m t h e t o p d o w n , a n d w i t h the p r i n c i p l e o f a l w a y s c h o o s i n g t h e f i r s t p o s s i b l e a l t e rn at iv e. W h e n I c o n s i d e r e d t h e s l o w n e s s o f V e r s i o n 1 a n d a l s o r e a l i z e d t h a t w h a t t h e s e m a n t i c p a r t of it d i d w a s l a r g e l y r e p e a t i n g t h e s y n t a c t i c a n a l y s i s of t h e s e n t e n c e , it a p p e a r e d to m e t h a t it m i g h t be f r u i t f u l to t r y a n d b u i l d a s y s t e m w h e r e s y n t a c t i c an d s e m a n t i c a n a l y s i s w o u l d be d o n e in an i n t e g r a t e d fashion. Th i s , h o w e v e r , p u t s t r o n g e r d e m a n d s o n t h e p a r s i n g me ch an is m, si nc e it r e q u i r e d a m o r e i n t e l l i g e n t w a y of h a n d l i n g s t r u c t u r a l a m b i g u i t i e s .
is m o r e o r l e s s l i n e a r , w h e r e a s t h e p a r s i n g t i m e p e r w o r d in V e r s i o n 1 g r e w v e r y r a p i d l y w i t h the l e ng th of sentences.
Th e s y n t a c t i c a n a l y s i s in V e r s i o n 2 is d o n e a c c o r d i n g t o th e f o l l o w i n g p r i n c i p l e s :
(i) t h e o u t p u t is a L I S P s t r u c t u r e w h i c h c a n be c h a r a c t e r i z e d as an ' a l m o s t u n l a b e l l e d b r a c k e t i n g ' , tliat is, w i t h v e r y f e w except io ns , t h e s y n t a c t i c c a t e g o r y o f a c o n s t i t u e n t (which h a s t h e f o r m o f a li st ) is n o t e x p l i c i t l y m a r k e d b u t h a s to be d e d u c e d f r o m t h e l e x i c a l c a t e g o r y of i t s 'head', t h a t is the firs t m e m b e r (CAK; of the list
(ii) p a r s i n g is d o n e f r o m l e f t to r i g h t in a m o r e o r l e s s d e t e r m i n i s t i c w a y
(iii) the fact th at the c a t e g o r y of a c o n s t i t u e n t is in g e n e r a l k n o w n w h e n y o u h a v e i d e n t i f i e d i t s h e a d o r i t s f i r s t w o r d (which is o f t e n the s a m e thing) is s y s t e m a t i c a l l y e x p l o i t e d in p r e d i c t i n g w h a t c o m e s ne xt
(iv) b a c k t r a c k i n g i s m a d e b y a s y s t e m a t i c u s e o f l o c a l p a r a m e t e r s o f L I S P f u n c t i o n s : e v e r y t i m e a n e w w o r d is p a r s e d a ca ll is m a d e to a f u n c t i o n a n d t h e p a r t i a l s t r u c t u r e b u i l t so far is p a s s e d t o t h a t f u n c t i o n a s a p a r a m e t e r - if t h e c o n t i n u e d p a r s e d o es not succeed, one a u t o m a t i c a l l y r e t u r n s to the p r e v i o u s state
(v) a t a n y p o i n t in t h e p a r s i n g p r o c e s s , t h e p a r t i a l a n a l y s i s a r r i v e d a t s o f a r is r e p r e s e n t e d as a s i n g l e s t a c k o f ' a c t i v e co n s t i t u e n t s ' ( c a l l e d t h e A C T 1 V E S T A C K ) , t h a t is, c o n s t i t u e n t s t h at h a v e n o t y e t b e e n f i n i s h e d . T o s h o w w h a t t h e p a r s i n g of a s e n t e n c e m a y l o o k like, w e s h o w t h e s u c c e s s i v e s t a g e s of t h e p a r s i n g o f (2) in (3).
52-( 2 ) J o h n b e l i e v e s t h a t Mary l o v e s B i l l
(3)
(expression to be parsed:) (ACTIVESTACK:)
1: John b e lie v e s th a t Mary lo v e s B i l l NIL
2: b e lie v e s t h a t Mary l o v e s B i l l ((John) VP (S))
3: that Mary l o v e s B i l l (NP (believe -s) (S (John)))
4: Mary l o v e s B i l l (s (that) (believe -s) (S (John)))
5: lo v e s B i l l ((Mary) VP (S) (that)(bel ieve -s) (S (John)))
6: B i l l (NP (love -s) (S (Mary)) (that) (believe -s) (S (John)))
7: NIL ((Bill) (love -s) (S (Mary)) (that) (believe -s) (S (John))) (close all constituents)
(S (John) (believe -s (that (S (Mary) (love -s (Bill))))))
The a s s i g n m e n t of r e f e r e n t s to H P s is d o ne d a r i n g the s y n t a c t i c analysis, m o r e s p e c i f i c a l l y , w h e n t h e n o u n p h r a s e in q u e s t i o n is 'closed', i.e. m o v e d o f f t h e s t a c k o f a c t i v e c o n s t i t u e n t s . W h e n a r e f e r e n t is a s s i g n e d t o a n o u n p h r a s e , a ' d o t t e d pair' r e p r e s e n t i n g t h e r e f e r e n t is a d d e d to t h e l i s t w h i c h r e p r e s e n t s the c o n s t i t u e n t in t h e s t r u c t u r e . A t p r e s e n t , t h e s y s t e m c a n h a n d l e t h r e e k i n d s o f M P s: p r o p e r n a m e s , b o u n d p r o n o u n s , a n d NPs w i t h a p o s s e s s i v e in the d e t e r m i n e r slot. For p r o p e r nouns, the a s s i g n m e n t p r o c e s s is t r i v i a l : t h e p r o p e r n a m e i t s e l f is u s e d as a r e f e r e n c e indicator. Thus, tlie LI SP e x p r e s s i o n to the le ft o f t h e a r r o w is c o n v e r t e d i n t o t h e o n e t o t h e r i g h t o f t h e a r r o w :
(4) ( J o h n ) ---> ( J o h n (REF. J o h n ) )
For N P s w i t h a p o s s e s s i v e d e t e r m i n e r , t h e p r i n c i p l e is a l s o s l i g h t l y a d hoc: t h e r e f e r e n t o f t h e p o s s e s s i v e e x p r e s s i o n is first d e t e r m i n e d , t h e n t h e p r o p e r t y l i s t of t h a t r e f e r e n t is e x a m i n e d to see if th er e is s o m e p r o p e r t y w h i c h c o i n c i d e s w i t h the h e a d n o u n o f t h e NP: in t h a t case, t h e v a l u e o f t h a t p r o p e r t y b e c o m e s the r e f e r e n t of the w h o l e NP. For instance, if we h a v e t h e N P J o h n ^ s w i f e a n d w e f i n d t h e i t e m ( w i f e . M a r y ) o n J o h n ' s p r o p e r t y list, t h e n the r e f e r e n t of John's w i f e is t a k e n
to be M a r y .
The m o s t i n t e r e s t i n g p a r t of the r e f e r e n t a s s i g n m e n t p r o c e d u r e is t h a t w h i c h a s s i g n s a n t e c e d e n t s a n d r e f e r e n t s t o b o u n d pronouns. T h e a s s u m p t i o n is t h a t t h e a n t e c e d e n t o f a b o u n d p r o n o u n is t o b e f o u n d a m o n g t h e N P s t h a t c - c o m m a n d it. A c c o r d i n g to the c u r r e n t defi ni ti on , a no de x c - c o m m a n d s a node y if a n d o n l y if t h e n o d e t h a t i m m e d i a t e l y d o m i n a t e s x a l s o d o m i n a t e s In t h e p r e s e n t s y s t e m , tlie c - c o m m a n d e r s o f a n N P th at is b e i n g ' c lo se d' a r e a l w a y s p r e c i s e l y t h o s e N P s t h a t a r e i m m e d i a t e c o n s t i t u e n t s o f t h e m e m b e r s o f t h e A C T I V E S T A C K . F o r instance, w h e n t h e N P B£_l^ i n (2) a b o v e is c l o s e d , t h e A C T I V E S T A C K looks as follows:
(5) (l o v e -s) (S ( M a r y ) ) (that) ( b e l i e v e -s) (S (John)))
T h e c - c o m m a n d e r s in (b) are thus M a r y an d J o h n .
Th is m a k e s it p o s s i b l e t o f o r m u l a t e a r e l a t i v e l y s i m p l e a l g o r i t h m for f i nd in g the p o s s i b l e ante ce de nt s. In a d d i t i o n to a s s i g n i n g a r e f e r e n t t o a p r o n o u n , t h e a l g o r i t h m a l s o s t o r e s the d i s t a n c e (in nodes) b e t w e e n the p r o n o u n and its antecedent. The p o i n t of th is w i l l b e c o m e c l e a r later.
r o u t i n e t h a t p u t s i n a r e f e r e n t i a l i n d e x o r t l i e l i k e .
In m u L I S P f o r m a l i s m the m a i n a n t e c e n d e n t - f i n d i n g f u n c t i o n looks as f o l l o w s (some i r r e l e v a n t d e t a i l s h a v e b e e n left out):
-54-( 6)
(DEFUN ANTECEDENT (LAMBDA (X Y XNP XNODE DIST)
(SETQ Y (CDR ACTIVESTACK)) Define Y as the ACTIVESTACK minus the NP under consideration.
(SETQ DIST 0) Set the variable DIST to 0.
(LOOP Repeat unti l Y is e m p t y or a n t e c e d e n t is f o u nd: ((NULL Y) NIL)
(SETQ XNODE (POP Y)) Set XNODE to next member of Y.
(SETQ XNP (FIRSTNP XNODE)) Find the first NP in XNODE: call it XNP. ((AND
(MEMBER (CAR X) REFLPROLIST) If the pronoun is reflexive and
(EQ (CAT XNODE) S) ) XNODE is a sentence, then
(PUT X 'ANTEC-DIST DIST) set the antecedent-distance to DIST and (AGREE X XNP) ) the a n tece de nt to XNP, if it agrees w i t h
the pronoun, else to NIL,
((AND (if the pronoun is non-reflexive:)
(AGREE X XNP) if XNP agrees with the pronoun then
(NOT (AND unless the pronoun is non-possessive
(NOT (POSSESSIVE X)) and
(EQ (GET XNP REF) (GET (SUBJECT) REF)) )) ) XNP is coreferent with the subject of the sentence, (PUT X 'ANTEC-DIST DIST)then set the antecedent-distance to DIST
XNP ) and the antecedent to XNP.
XNP )
(SETQ DIST (ADDl DIST)) ) ) )) Add 1 to DIST.
T h i s is c e r t a i n l y a s i m p l i f i e d ru l e ; fo r i n s t a n c e , it a s s u m e s t h a t t h e a n t e c e d e n t o f a r e f l e x i v e is a l w a y s t h e s u b j e c t . However, in m o s t s i m p l e cases, it a s s i g n s the c l o s e s t p o s s i b l e a n t e c e d e n t to a n y b o u n d pr onoun.
c o n t a i n s the p r o n o u n is r e t r i e v e d later on. W e shall i l l u s t r a t e w h a t t h i s m e a n s by l o o k i n g a t t h e w a y in w h i c h t h e p r o g r a m h a n d i e s 'sloppy identity'. C o n s i d e r the a g a i n the e x a m p l e f r o m the b e g i n n i n g of the paper;
(7) J o h n loves his w i f e and so d o es Bill
At p r e s e n t , t h e p r o g r a m is o n l y a b l e t o h a n d l e a s o m e w h a t u n i d i o m a t i c p a r a p h r a s e of (7);
(8) J o h n loves h i s w i f e and Bill too.
B a si ca ll y, t h e f o l l o w i n g is w h a t h a p p e n s w h e n (8) is i n t e r p r e t e d b y t h e s y s t e m ; F i r s t , t h e c l a u s e J o h n l o v e s h i s w i fe is p a r s e d . A s s u m i n g t h a t t h e s y s t e m k n o w s t h a t M a r y is J o h n ' s w i f e , it w i l l a s s i g n M a r y as a r e f e r e n t to h i s w i f e . Then, t h e r e d u c e d c l a u s e B i l l t o o is p a r s e d . A f t e r t h e s u b j e c t N P Bill the s y s t e m e x p e c t s a v e r b phrase; it ta ke s the p a r t i c l e too as a si g n a l of an e l l i p t i c a l VP. E v e r y t i m e a VP is parsed, it b e c o m e s t h e v a l u e o f t h e v a r i a b l e L A S T V P ; in t h i s ca s e , L A S T V P is loves his w i f e . Th e p a r s e d v e r s i o n of t h is e x p r e s s i o n is n o w c o p i e d i n t o t h e p l a c e w h e r e t h e VP s h o u l d o c c u r in t h e e l l i p t i c a l s e n t e n c e . W h e n t h i s h a p p e n s , t h e N P h i s w i f e is a g a i n s u b j e c t e d to the r e f e r e n c e a s s i g n m e n t p r o c e s s - h o w e v e r , s i n c e it is the s e c o n d time, the a n t e c e d e n t is foun d not by the f u n c t i o n A N T E C E D E N T b u t by a n o t h e r c a l l e d F I N D - A N T E C E D E N T - AGAIN. T h i s f u n c t i o n l o o k s a t t h e a n t e c e d e n t d i s t a n c e a s s o c i a t e d w i t h t h e p r o n o u n h i s a n d t r i e s t o f i n d t h e N P a t t h e c o r r e s p o n d i n g p l a c e in t h e tree. In t h i s case , it is B i l l , so the r e f e r e n t of his w i f e is n o w t a k e n to be Bill's wife.
V e r s i o n 2 h a s n o t y e t b e e n d e v e l o p e d s o f a r t h a t it c a n t a k e care o f t h e o t h e r p r o b l e m a t i c c a s e s o f b o u n d p r o n o u n s , b u t in p r i n c i p l e s i m i l a r m e c h a n i s m s a s t h e o n e m e n t i o n e d s h o u l d be s u f f i c i e n t t o s o l v e t h e p r o b l e m s , as w a s d e m o n s t r a t e d b y V e r s i o n 1. T h e p o i n t is t h a t the a n t e c e d e n t d i s t a n c e p a r a m e t e r a p p r o a c h is i n h e r e n t l y m o r e p o w e r f u l t h a n t h e c o m m o n w a y o f d i s p l a y i n g c o r e f e r e n c e r e l a t i o n s , viz. b y r e f e r e n t i a l i n d i c e s or m u l t i p l e o c c u r r e n c e s of th e s a m e v a r i a b l e letter, in t h a t it
-The a b o v e a c c o u n t h a s b e e n l a c k i n g in e x p l i c i t n e s s in v a r i o u s ways. T h e r e a r e t w o r e a s o n s for this: the r a t h e r e a r l y stag e of d e v e l o p m e n t of the p r o g r a m a n d the l i m i t e d s p a c e avai la bl e. The l o n g - r a n g e a i m o f t h e u n d e r t a k i n g is t o p r o v i d e a s m a l l y e t p o w e r f u l ' m o d u l e ' f o r p r o c e s s i n g n a t u r a l l a n g u a g e s s e n t e n c e s a n d texts, w h e r e the p r o n o u n i n t e r p r e t a t i o n m e c h a n i s m w i l l o n l y be a s m a l l p a r t . H o p e f u l l y , t h e w o r k o n t h e ' m o d u l e ' w i l l be p o s s i b l e to s h e d light on s o m e q u e s t i o n s of g e n e r a l t h e o r e t i c a l i n t e r e s t .
h a s a m e a n i n g f u l i n t e r p r e t a t i o n a l s o o u t o f c o n t e x t .
R E F E R E N C E S
Dahl, Ö. 1 9 83 a. O n t h e n a t u r e o f b o u n d p r o n o u n s . P I L U S 48, Dept, of L i n g u i s t i c s , Univ. of S t o c k h o l m .
Dahl, Ö. 1 9 8 3 b . B o u n d p r o n o u n s in a n i n t e g r a t e d p r o c e s s m o d e l . In F. K a r l s s o n , ed.. P a p e r s f r o m t h e S e v e n t h S c a n d i n a v i a n C o n f e r e n c e o f L i n g u i s t i c s . U n i v e r s i t y o f H e l s i n k i , Dept, o f G e n e r a l Li ng ui st ic s.
Dahl, U. 1985. S y n t a c t i c P a r s i n g o n a M i c r o c o m p u t e r . In S. B ä c k m a n a n d G. K j e l l m e r , e d s . ,
L i t e r a t u r e £ £ £ £ ^ n t e d t o A ^ v a r EJii:£
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[^an. G o t h e n b u r g S t u d i e s in E n g l i s h 60. Gö te bo rg : A c t a U n i v e r s i t a t i s G o t h o b u r g e n s i s .Engdahl, E. 1985. T h e S y n t a x a n d S e m a n t i c s o f Q u e s t i o n s w i t h S p e c i a l R e f e r e n c e to S w e d i s h . D o r d r e c h t : Re id el .
