BACKGROUND: THE SAM SYSTEM
FOUL-UP
A program t h a t f i g u r e s out
meanings of words f r o m c o n t e x t
1.
Richard H. Granger, J r .
D e p a r t m e n t o f Computer S c i e n c e
Yale U n i v e r s i t y
New H a v e n , C o n n e c t i c u t 06520
The i n f e r e n c i n g t a s k o f f i g u r i n g
out
words
from
c o n t e x t is implemented in the presence of a
l a r g e database of w o r l d knowledge.
The
program
does
not
r e q u i r e i n t e r a c t i o n w i t h the u s e r , b u t
rather
uses
internal
parser
expectations
and
knowledge
embodied
in
scripts
to
figure
out
likely definitions
for
unknown
words,
and
to
create
context-specific
definitions
for
such
words.
INTRODUCTION
" I woke u p y e s t e r d a y , t u r n e d
off
my
alarm
clock,
took a
shower,
and
cooked
m y s e l f two
grimps f o r b r e a k f a s t . " What's a
grimp?
On
the
basis
o f t h i s one s t o r y , i t c a n ' t b e d e t e r m i n e d .
I t seems, h o w e v e r , t h a t a g r i m p must b e some s o r t
of
food,
in
order
f o r i t t o make sense i n t h e
context of the s t o r y .
The f a c t t h a t a g r i m p i s a
physical
o b j e c t , and t h u s a n o u n , i s s u c h a n a t u r a l inference that it is taken f o r g r a n t e d .
The
process by which these c o n c l u s i o n s about " g r i m p s "
a r e a r r i v e d a t i s commonly c a l l e d " f i g u r i n g o u t a
word
from c o n t e x t " .
This process is modelled by
t h e FOUL-UP p r o g r a m .
FOUL-UP e n a b l e s
the
SAM
system
(see
Schank e t a l , 1975) t o r e a d s t o r i e s
s u c h as t h e a b o v e , and to make an e d u c a t e d
guess
as
t o what a " g r i m p " s h o u l d b e .
The m e t h o d s emp l o y e d b y FOUL-UP a r e based o n
intuitions
about
how
the analogous tasks a r e performed by p e o p l e .
Thus i t s a b i l i t i e s and l i m i t a t i o n s s h o u l d r e f l e c t
those
of people.
FOUL-UP p r o v i d e s t h e f o l l o w i n g
p u r e l y p r a c t i c a l a s s e t s t o SAM:
(1)
it
allows
reading
of
unedited
newspaper
stories,
which
t y p i c a l l y assume a l a r g e v o c a b u l a r y , and
(2)
it
c r e a t e s d i c t i o n a r y d e f i n i t i o n s f o r unknown w o r d s ,
w h i c h w o u l d o t h e r w i s e have t o b e done b y hand f o r
every
word
in
SAM's d i c t i o n a r y .
Other n a t u r a l
l a n g u a g e - b a s e d s y s t e m s have
attempted
to
allow
for
t h e o c c u r a n c e o f unknown w o r d s d u r i n g u n d e r standing,
( s e e Woods &
Kaplan,
1971,
and
Winograd, 1971), but these u s u a l l y I n v o l v e having
the user a i d the system in d e f i n i n g
the
unknown
word.
FOUL-UP
acts
independently of the u s e r ,
t a k i n g i t s i n f o r m a t i o n e n t i r e l y f r o m the i n t e r n a l
representation
of
knowledge
t h a t e x i s t s in the
SAM s y s t e m .
T h i s work was s u p p o r t e d i n p a r t b y
the
Adv a n c e d R e s e a r c h P r o j e c t s Agency o f t h e D e p a r t m e n t
o f D e f e n s e and m o n i t o r e d u n d e r t h e O f f i c e o f
Nav a l R e s e a r c h under c o n t r a c t N 0 0 0 1 4 - 7 5 - C - 1 1 1 1 .
Natural
O v e r v i e w of SAM
FOUL-UP was d e s i g n e d a s a n i n t e g r a l p a r t
of
the
SAM
system,
so
any d i s c u s s i o n o f i t s p e r f o r m a n c e w i l l i n v o l v e some k n o w l e d g e o f
SAM
itself.
The
SAM s y s t e m e x i s t s a s t h r e e main modu l e s , ELI
(English
Language
Interpreter),
TOK
(Tokenizer)
and
APPLY
(Script Applier).
Every
i n p u t sentence is f i r s t
parsed
by
ELI
into
a
Conceptual
Dependency (CD) r e p r e s e n t a t i o n o f t h e
sentence.
Then TOK i n c o r p o r a t e s s p e c i f i c o b j e c t s
into
memory
p o i n t e r s , f o r use i n d i s a m b i g u a t i n g
later
anaphoric
references.
Finally,
APPLY,
which
contains
the
scripts,
a t t e m p t s t o match
every CD i n t o the c u r r e n t l y a c t i v e s c r i p t .
When a n u n d e f i n e d word l i k e
"grimp"
occurs
in
a s e n t e n c e , i t i s ELI t h a t w i l l f i r s t e n c o u n t e r i t , and i t w i l l " f o u l
up"
the
normal
processing
protocol,
by
calling
FOUL-UP w i t h t h e
w o r d t h a t caused t h e t r o u b l e .
Ideally,
at
this
point
FOUL-UP s h o u l d a t t e m p t t o g u e s s t h e w o r d ' s
meaning b y l o o k i n g i n t o the c o n t e x t o f
the
surrounding s c r i p t ( e . g .
"the Get-Up-In-The-Morning
s c r i p t " ) , t o see what o b j e c t was e x p e c t e d i n
the
position
that
t h e unknown word " g r i m p " o c c u r r e d
in ( s e e R i e s b e c k &. S c h a n k , 1 9 7 6 ) .
The " p o s i t i o n "
o f t h e unknown w o r d does n o t r e f e r t o t h e s u r f a c e
position
in
the.
sentence,
but
rather
the
conceptual
slot
i n t h e CD, c o r r e s p o n d i n g t o t h e
unknown w o r d ' s m e a n i n g .
This process
cannot
be
immediately invoked at the time of E L I ' s f o u l - u p ,
f o r three reasons:
(1) a l t h o u g h the s c r i p t has a l r e a d y been e n t e r e d ,
we have not y e t APPLYed the CD c o n c e p t u a l i z a t i o n
f o r the c u r r e n t sentence i n t o the s c r i p t ,
so
we
therefore
do
not
yet
have
the s p e c i f i c l o c a l
s c r i p t i n f o r m a t i o n t h a t wc need to f i n d
the
expected meaning of the unknown word;
(2) ELI has
not yet f i n i s h e d i t s
parse
of
the
current
sentence,
so there does not yet e x i s t a
complete CD to match i n t o the s c r i p t ;
(3) s i n c e ELI has not f i n i s h e d the parse, we do
not
yet know which s l o t of the CD w i l l be f i l l e d
by the unknown word, so we could not f i n d
the
corresponding
slot
in
the
script,
even if we
could match the CD i n t o the s c r i p t .
FOUL-UP c o u l d , of c o u r s e ,
attempt
to
comp l e t e the CD by I t s e l f , use E L I ' s e x p e c t a t i o n s to
f i n d the c o r r e c t s l o t f o r the unknown word, match
the CD i n t o the s c r i p t , and thus guess a p o s s i b l e
meaning f o r the word.
However, w i t h a few modif i c a t i o n s , the normal o p e r a t i o n of the SAM system
w i l l do a l l t h i s f o r us.
Thus
FOUL-UP makes a
note
of
the c u r r e n t ELI e x p e c t a t i o n s f o r f u t u r e
r e f e r e n c e , and
then
proceeds
to pass back a
" p l a c e - h o l d e r " f o r ELI to use In c o n s t r u c t i n g the
c u r r e n t CD.
T h i s p l a c e - h o l d e r is so
constructed
as
to
s a t i s f y any c u r r e n t ELI e x p e c t a t i o n , so
when c o n t r o l r e t u r n s t o E L I ,
it
will
automatically
put
this
p l a c e - h o l d e r i n t o the s l o t t h a t
the unknown word belongs
in.
The e x p e c t a t i o n s
that
ELI
has f o r such incoming words c o n s i s t of
both s y n t a c t i c and semantic r u l e s .
Thus ELI
ex-
LanjuiaKe-9:
172
Grander
pects
the
incoming
word to be a n o u n , and a PP
( " p i c t u r e p r o d u c e r " ) (see Schank, 1 9 7 5 ) .
All
of
ELI's
expectations
are
such
t h a t the expected
word w i l l f i t a p p r o p r i a t e l y i n t o the sentence
in
which it appears.
ELI i s n o t capable o f e n s u r i n g
t h a t t h e word w i l l
fit
appropriately
into
the
story,
or
any
other context l a r g e r than a sentence.
It is the
script
applier
(APPLY)
that
checks
l a r g e r c o n t e x t s , s p e c i f i c a l l y , the s c r i p t
context of the s t o r y .
We w i l l c a l l ELI's
expectations
" i n t r a - s e n t e n c e " e x p e c t a t i o n s , w h i l e APPLY's are c a l l e d " i n t e r - s e n t e n c e " e x p e c t a t i o n s .
2.
Script
Context
Once c o n t r o l has b e e n t h u s r e t u r n e d t o
ELI,
and i t h a s i n c o r p o r a t e d t h e p l a c e - h o l d e r i n t o t h e
CD, t h e n c o n t r o l c o n t i n u e s t h r o u g h
the
rest
of
the
system
a s t h o u g h t h e f o u l - u p had n o t o c c u r red.
M e a n w h i l e , FOUL-UP has n o t i f i e d t h e
system
that
a p l a c e - h o l d e r e x i s t s i n t h e c u r r e n t CD, s o
t h a t i t can b e h a n d l e d b y each module
correctly.
Thus
TOK
is
able
to t o k e n i z e the p l a c e - h o l d e r
c o r r e c t l y , and t h e n APPLY i s c a l l e d .
APPLY
now
has
a CD c o n t a i n i n g a p l a c e - h o l d e r to a t t e m p t to
match i n t o the s c r i p t .
It
performs
a
partial
pattern-match,
a l l o w i n g f o r the e x i s t e n c e o f the
p l a c e - h o l d e r , and n o t i n g w h i c h
script-slot
corr e s p o n d s t o t h e p l a c e - h o l d e r s l o t i n t h e CD.
A t t h i s p o i n t i n the p r o c e s s i n g ,
all
three
of
the
above n e c e s s a r y p r o c e s s e s have been p e r formed:
a s l o t has been found
for
the
unknown
w o r d , a CD h a s been c r e a t e d , and t h a t CD h a s b e e n
matched i n t o the s c r i p t .
Now FOUL-UP
is
called
back
i n b y APPLY.
S i n c e the CD has been matched
I n t o the s c r i p t , t h e r e i s a c o n c e p t u a l i z a t i o n
in
the
script
which
corresponds
t o t h e CD.
This
script conceptualization contains a t t r i b u t e s
ass o c i a t e d w i t h e a c h s l o t i n t h e CD, i n c l u d i n g t h a t
of the p l a c e - h o l d e r c o r r e s p o n d i n g to the
unknown
word.
These
attributes
a r e o r d i n a r i l y used t o
ensure t h a t the c o n c e p t u a l i z a t i o n appearing in
a
given
s l o t i s compatible w i t h the l a r g e r context
surrounding that s l o t .
Thus,
for
example,
it
expects
to
see
a food as the o b j e c t of the i n s t r u m e n t a l s c r i p t "cook" i n the
optional
"MakeBreakfast"
scene
of the "Get-Up-In-The-Morning"
script.
FOUL-UP l o o k s a t t h e s l o t i n t h e
script
corresponding
t o t h e p l a c e - h o l d e r , and n o t e s t h e
a t t r i b u t e s associated with that
slot.
Then
it
compares
these
inter-sentence expectations with
t h e i n t r a - s e n t e n c e e x p e c t a t i o n s t h a t E L I had
for
the
slot.
I t combines the c o m p a t i b l e s e t o f a t t r i b u t e s i n t o a d i c t i o n a r y e n t r y f o r the
unknown
word,
and
writes
this
d e f i n i t i o n onto a f i l e ,
c a l l e d the P r i m e r , which the parser from then
on
uses
as
part
of
i t ' s dictionary.
Thus i f t h e
word " g r i m p " appears a g a i n in a s i m i l a r
context,
SAM w i l l " r e m e m b e r " i t a s b e i n g a noun and a PP,
and
having
appeared
before
as
a
food
in
a
"Breakfast"
story,
so it w i l l successfully f i l l
t h e a p p r o p r i a t e s l o t i n a CD, and w i l l
correctly
match i n t o the s c r i p t .
OPERATION OF THE FOUL-UP PROGRAM
1"
A n n o t a t e ^ Run-Time O u t p u t
The f o l l o w i n g
represents
actual
annotated
run-time
output
of
t h e FOUL-UP p r o g r a m r u n n i n g
c o n c u r r e n t l y w i t h t h e SAM s y s t e m .
The
input
to
the system i s the s i m p l e s t o r y :
car
" F r i d a y , a c a r swerved o f f
struck an elm."
Route
69.
The
I t i s g i v e n t h a t t h e s y s t e m knows a l l
the
words
in
the s t o r y except " e l m " .
The APPLY m o d u l e has
a s c r i p t c a l l e d $Vehaccident, which contains
the
necessary
r e a l - w o r l d knowledge about v e h i c l e a c c i d e n t s (see C u l l i n g f o r d , 1977).
The f i r s t
sentence
in
the
story
goes
s t r a i g h t through the
system w i t h o u t i n c i d e n t , s i n c e a l l the words
are
defined.
The m a j o r e f f e c t o f t h e f i r s t s e n t e n c e
i s t o a c t i v a t e the s c r i p t " $ V e h a c c i d e n t " ,
recogn i z i n g t h a t t h i s sentence i n d i c a t e s the beginning
of an a c c i d e n t s t o r y .
The f o l l o w i n g o u t p u t comes
f r o m t h e p r o c e s s i n g o f t h e second s e n t e n c e .
>EL1<
The c a r s t r u c k a n e l m .
ELI
has n o d e f i n i t i o n f o r t h e word
"elm".
>FOUL-UP<
FOUL-UP n o t i f y i n g TOK & APPLY of unknown w o r d ELM
O b t a i n i n g CONINFO and SYNINFO f r o m E L I
R e t u r n i n g t o E L I w i t h bogus p l a c e - h o l d e r :
(//BOGUS LEXVAL (ELM) REF ( D E F ) )
FOUL-UP h a s now c r e a t e d a p l a c e - h o l d e r f o r t h e
unknown w o r d .
I t has a l s o s t o r e d c o n c e p t u a l &
s y n t a c t i c e x p e c t a t i o n s a s s o c i a t e d w i t h the
word.
These w i l l b e used l a t e r , d u r i n g FOULUP's c o n s t r u c t i o n o f a d e f i n i t i o n f o r " e l m " .
>EL1<
((ACTOR (//PHYSOBJ TYPE (*CAR*) REF (DEF)) <->
(*PROPEL*) OBJECT (//BOGUS LEXVAL (ELM) REF
( I N D E F ) ) ) TIME ( T I M 2 ) )
E L I c r e a t e s a s much o f a C D f r o m t h i s s e n t e n c e
a s i t c a n , i n c l u d i n g the p l a c e - h o l d e r from
FOUL-UP.
A l l i t r e a l l y knows i s : t h e a c t o r
i s a c a r , and " s t r i k e " i n d i c a t e s a " p r o p e l " .
>APPLY<
APPLY l o c a t i n g unknown PPs in t o p - l e v e l TOK a t o m
Bound s c r i p t v a r i a b l e : ^OBSTRUCTION to BOGO
APPLY d o e s a p a r t i a l p a t t e r n - m a t c h , and p o i n t s
out s l o t i n the match which c o r r e s p o n d s t o
p l a c e - h o l d e r i n the c o n c e p t u a l i z a t i o n from ELI
>FOUL-UP<
A t t e m p t i n g to p a r t i a l l y understand E1M
ELI e x p e c t a t i o n s were:
((AND (OR (POBJ) (HUMAN)) (NP)))
E L I ' s e x p e c t a t i o n s are l o o k i n g f o r e i t h e r a
p h y s i c a l o b j e c t or a human, and a noun phrase.
These are simply i n t r a - s e n t e n c e e x p e c t a t i o n s .
Natural
Lan*ua*e-9: Hran*er
173
APPLY e x p e c t a t i o n s w e r e :
( S c r i p t r o l e SUBSTRUCTION
i n $VEHACCIDENT)
APPLYs i n t e r - s e n t e n c e e x p e c t a t i o n s e x p e c t t h i s
s l o t f i l l e r to play the r o l e of Substruction in
t h e s c r i p t $ V e h a c c i d e n t . Now FOUL-UP w i l l p i c k
a c o m p a t i b l e s u b s e t o f t h e s e e x p e c t a t i o n s , and
c o n s t r u c t a d e f i n i t i o n f o r t h e word " e l m " .
FOUL-UP g u e s s e s t h e f o l l o w i n g d e f i n i t i o n f o r ELM:
(PROG N I L
(DEFPROP ELM T PP)
(1)
(DEFPROP ELM *PHYSOBJ* I S A )
(DEFPROP ELM T NOUN)
(DEC ELM
(2)
(T ( N I L ((SHAPE N P ) )
NIL
(QUOTE (NOUNGROUP))
(QUOTE (STARTNOUNGR))))))
(DENG ELM
(3)
(VAL (//PHYSOBJ FUNCTION
( ( < - > ($VEHACCIDENT
OBSTRUCTION MODFOCUS)))))
(MARKER NOUN))
This d e f i n i t i o n is in three p a r t s :
( 1 ) p u t t h e p r o p e r t i e s PP, PHYSOBJ, and
NOUN o n t h e w o r d , t o f i l l E L I e x p e c t a t i o n s ;
( 2 ) a t t a c h n o u n - g r o u p e r (see Gershman, 1 9 7 7 ) ;
(3) d e f i n e the conceptual s t r u c t u r e t h a t the
w o r d w i l l b u i l d i n a CD, s a y i n g t h a t i t i s o f
t h e c o n c e p t u a l c l a s s //PHYSOBJ & p l a y s t h e r o l e
of an o b s t r u c t i o n in a car accident s c r i p t .
2
-
Note:
Context-Specificity of Definitions
A f t e r the above run of
SAM,
the
resulting
dictionary
definition
for
" e l m " w o u l d become a
p e r m a n e n t p a r t o f SAM's d i c t i o n a r y .
Let
us
now
t a k e a c l o s e r l o o k a t e x a c t l y what i n f o r m a t i o n i s
contained i n the d e f i n i t i o n f o r " e l m " .
At
this
p o i n t , " e l m " has b e e n d e f i n e d a s n o more n o r l e s s
than a p h y s i c a l o b j e c t which plays the r o l e of an
obstruction in an accident s c r i p t .
T h i s does n o t
seem t o r e f l e c t t h e
true
meaning
of
the
word
" e l m " a t a l l , b u t r a t h e r seems o n l y t o embody t h e
m e a n i n g o f t h e s p e c i f i c use o f t h e w o r d
in
this
one s t o r y .
M o s t w o r d s h a v e more t h a n one m e a n i n g
or f u n c t i o n , depending on the c o n t e x t they appear
in.
S i n c e FOUL-UP i s g i v e n o n l y one c o n t e x t , t h e
s t o r y i t s e l f , i t can o n l y guess the s i n g l e
meaning
apropos o f t h a t s i n g l e s t o r y .
The r e a s o n i t
seems l i k e a p o o r d e f i n i t i o n t o u s i s b e c a u s e
we
in
fact
h a v e s e e n t h e w o r d " e l m " many t i m e s b e f o r e , and a l r e a d y h a v e
rather
complete
definit i o n s f o r i t , i n many c o n t e x t s .
Note t h a t i f the
s a m p l e r u n had c o n t a i n e d t h e w o r d
"bogosity"
in
the
place
of
"elm",
("The
car
struck
a bog o s i t y " ) , then
FOUL-UP's
definition
may
still
have
seemed
sketchy,
but
it would be about as
much a s a p e r s o n c o u l d have f i g u r e d o u t f r o m j u s t
that
one
exposure to the word.
From t h e s i n g l e
use o f " e l m " ( o r " b o g o s i t y " )
in
the
story,
it
could
be d e f i n e d as a t r e e , or a telephone p o l e ,
o r one o f t h o s e l a r g e o r a n g e b a r r e l s
often
seen
by
the
s i d e o f t h e r o a d , o r any number o f o t h e r
things.
( I t m i g h t seem a t f i r s t t h a t
a
"bogosity"
could
be another car, or a person, instead
Natural
of an inanimate o b j e c t .
It
is
the
first
sentence,
"A
c a r swerved o f f Route 6 9 " t h a t causes
APPLY ( p e r h a p s p r e m a t u r e l y ) t o p r e f e r t h e o n e - c a r
accident
track,
in
which
b y s t a n d e r s and o t h e r
cars play no p a r t .
T h i s t r a c k i s assumed i n
the
absence
of
further
i n f o r m a t i o n . ) The o n l y c o n c r e t e t h i n g w e c a n say a b o u t a " b o g o s i t y " i s
the
w a y I t f u n c t i o n s i n t h e c o n t e x t o f t h e s c r i p t and
s t o r y i t appears i n .
That i s p r e c i s e l y
the
information
that
a
s c r i p t r o l e c o n v e y s i n t h e SAM
s y s t e m , and t h a t i s why FOUL-UP
defined
"bogosity"
i n terms o f a s c r i p t r o l e .
In fact, without
the inter-sentence expectations provided
by
the
script,
the
definition
would
have
been
even
weaker.
Only E L I ' s
intra-sentence
expectations
could
have
been
u s e d , and t h e d e f i n i t i o n w o u l d
have s i m p l y s a i d t h a t a n
"elm"
(or
"bogosity")
was a noun and a p h y s i c a l o b j e c t .
CATEGORIES OF FOUL-UPS
1.
Nouns
The FOUL-UP
program
works
solely
on
the
b a s i s o f i n t r a - and i n t e r - s e n t e n c e e x p e c t a t i o n s ,
b u i l t i n t o the parser
and
the
script
applier,
respectively.
Thus
any c o n c e p t u a l i z a t i o n w h i c h
c a n b e " e x p e c t e d " i n t h i s sense b y SAM s h o u l d
be
able
to
be
figured
out
from
context
by the
FOUL-UP p r o g r a m , and FOUL-UP s h o u l d
be
able
to
use
the
available
expectations
to c o n s t r u c t a
context-specific definition
for
the
word,
regardless
of
its
syntactic
or
semantic c l a s s .
However, t h e examples so
far
given
have
dealt
only
with
nouns.
I n t h i s s e c t i o n , Foul-Ups o f
o t h e r p a r t s o f speech w i l l b e
demonstrated,
and
it
will
be
shown
why e a c h d i f f e r e n t s y n t a c t i c
c l a s s p r e s e n t s i t s own u n i q u e d i f f i c u l t i e s t o t h e
FOUL-UP p r o g r a m .
2*
Anaphoric
Consider
References
the
simple
"A c a r swerved
struck a t r e e . "
story:
o f f Route
69.
The
flivver
A g a i n , w e assume a l l t h e w o r d s
are
defined
but
"flivver",
and we assume a r u n of t h e SAM s y s t e m
with this story as input.
Once a g a i n , t h e
first
sentence
presents
no
d i f f i c u l t y , and s e r v e s t o
invoke the s c r i p t c o n t e x t which i s necessary
for
setting
up
the i n t e r - s e n t e n c e expectations t h a t
FOUL-UP u s e s t o f i g u r e o u t
the
meaning
of
the
word.
The f l o w o f c o n t r o l o f t h e SAM r u n o f t h i s
sentence would be p r a c t i c a l l y i d e n t i c a l
to
that
of
the previous example.
ELI's expectations are
also s i m i l a r .
However,
the
inter-sentence
expectations
from
APPLY
are
of a d i f f e r e n t form
than those in the p r e v i o u s examples.
APPLY s t i l l
matches
the
CD f o r t h e second s e n t e n c e i n t o the
$ V e h a c c i d e n t s c r i p t , and s t i l l n o t e s t h e
attributes
o f the s l o t i n the s c r i p t c o n c e p t u a l i z a t i o n
c o r r e s p o n d i n g t o t h e p l a c e - h o l d e r s l o t i n t h e CD.
Those
attributes,
however, are no longer in the
form of
a
scriptrole
specification.
Instead,
associated
with
that
s l o t is a pointer, saying
t h a t the o b j e c t i n t h i s s l o t i s the
same
object
L a r m , a R e - 9 : Grander
17U
as
that
which appeared in the ACTOR s l o t of the
previous sentence.
T h i s is obvious when we
note
that
(1)
the
script
must know t h a t the o b j e c t
t h a t "swerves o f f the r o a d " i n a n a c c i d e n t i s the
same a s the o b j e c t t h a t l a t e r i s s a i d t o " s t r i k e "
s o m e t h i n g , and (2) SAM as a whole has
the capability
to
r e s o l v e g e n e r a l anaphoric r e f e r e n c e s .
Thus the second sentence in the s t o r y might
have
read s i m p l y " I t s t r u c k a t r e e " , and the anaphoric
r e f e r e n c e t o " i t " would b e r e s o l v e d a s " t h e
car"
t h a t appeared e a r l i e r .
When such a s i t u a t i o n o c c u r s , FOUL-UP
takes
advantage
of
the
pointer
to the r e f e r e n t , and
c o n s t r u c t s a d e f i n i t i o n based
on
this
pointer.
Such
a d e f i n i t i o n w i l l s i m p l y d u p l i c a t e the d e f i n i t i o n o f the word t h a t
filled
the
pointed-to
slot,
since
the unknown word i s presumably j u s t
a n o t h e r r e f e r e n c e to the same o b j e c t .
Thus,
the
definition
for
an
unknown word i n t h i s c i r c u m s t a n c e w i l l b e b e t t e r than the
"FUNCTION"
definition
of
"elm"
in
the p r e v i o u s example.
The
d e f i n i t i o n w i l l n o t s i m p l y g i v e the
function
of
the
word i n t h i s s c r i p t c o n t e x t , b u t r a t h e r w i l l
g i v e as complete a d e f i n i t i o n as e x i s t s
for
the
a l r e a d y d e f i n e d word " c a r " which the unknown word
i s a n anaphoric r e f e r e n c e t o .
Thus i n
this
example,
the d e f i n i t i o n c r e a t e d f o r " f l i v v e r " w i l l
be i d e n t i c a l to that for " c a r " :
(PROG N I L
(DEFPROP FLIVVER T PP)
(DEFPROP FLIVVER *PHYSOBJ* I S A )
(DEFPROP FLIVVER T NOUN)
(DEC FLIVVER (T ( N I L ((SHAPE N P ) ) N I L
(QUOTE (NOUNGROUP))
(QUOTE (STARTNOUNGR)))))
(DENG FLIVVER (VAL (//PHYSOBJ TYPE ( * C A R * ) ) )
(MARKER NOUN)))
The " T Y P E " s p e c i f i c a t i o n i n t h i s d e f i n i t i o n i s
a
reference
to the e n t r y "*CAR*" in a g e n e r a l d i c t i o n a r y which d e f i n e s a t t r i b u t e s of
generic
objects
such
as
cars,
and t h e i r u s e s i n c e r t a i n
contexts.
Thus a n y t h i n g
that
SAM knows
about
cars
i t now a l s o knows a b o u t f l i v v e r s .
Furthermore, i t i s l i k e l y t h a t the term " f l i v v e r " r e f e r s
to a s p e c i f i c subset of a l l c a r s , ( e . g .
"sedan",
"jeep").
Therefore,
the
term
is
saved,
and
FOUL-UP g i v e s t h e t e r m t o SAM's g e n e r a t o r p r o g r a m
( s e e G o l d m a n , 1975) t o use when r e f e r r i n g t o
the
car in t h i s s t o r y .
3.
Verbs
a.
Structure-builders vs.
Consider
into
the
Slot-fillers
story
"A c a r swerved
a tree."
o f f Route 6 9 .
The
car
hied
The unknown w o r d I n t h e
sentence,
("hied"),
is
recognized
as
being
a v e r b by ELI e x p e c t a t i o n s
and b y t h e f a c t t h a t i t e n d s i n
a
regular
verb
ending.
A
simple
place-holder
cannot
be
straightforwardly constructed for
the
word,
as
was t h e c a s e f o r n o u n s .
T h i s i s because the v e r b
Natural
i n a s e n t e n c e d e t e r m i n e s most o f t h e s t r u c t u r e o f
t h e C o n c e p t u a l Dependency r e p r e s e n t a t i o n f o r t h a t
sentence.
(see
Schank,
1975,
and
Riesbeck &
Schank,
1976)
Essentially,
the verb b u i l d s the
f r a m e w o r k o u t l i n e f o r t h e CD, and s e t s u p most o f
t h e e x p e c t a t i o n s a s t o w h a t w i l l come n e x t .
Then
t h e nouns b u i l d s t r u c t u r e s w h i c h
f i l l
slots
in
that
verb-frame.
This
can
be
I l l u s t r a t e d by
l o o k i n g at a parse of a
sample
sentence,
"John
went
to
New
York".
P a r t i a l l y " k e r n e l i z e d " CD
r e p r e s e n t a t i o n s f o r sentences w i l l
be
used
for
convenience, to f a c i l i t a t e references to s p e c i f i c
pieces of the
conceptualization.
The
internal
representations
o f CDs i n E L I a r e e n t i r e l y " k e r nelized" In this fashion.
A p a r t i a l l y kernelized
CD f o r the above sentence f o l l o w s :
Kl:
((ACTOR
(K2)
<->
(PTRANS)
OBJECT
K2:
(//PERSON FIRSTNAME
K3:
(//LOCALE STATE
( K 2 ) TO
(K3)))
(JOHN))
(*NY*))
The w o r d " w e n t " b u i l d s
the
structure
Kl,
with
empty
slots
in
the
ACTOR, OBJECT and T O p o s i tions.
The w o r d " J o h n " b u i l d s t h e s t r u c t u r e
K2,
which
s i m p l y f i l l s i n t h e ACTOR and OBJECT s l o t s
i n K l , a n d "New Y o r k " b u i l d s K 3 , w h i c h f i l l s
the
TO
slot
in
Kl.
A l t h o u g h t h e s e c o n s t r u c t s may
l o o k a b o u t t h e same s i z e and c o m p l e x i t y , I t i s i n
fact
Kl,
the v e r b - f r a m e , which d r i v e s the parse
of the r e s t of the
sentence.
The
verb
itself
sets
up
the
e x p e c t a t i o n s t h a t a //PERSON s h o u l d
l i l l t h e ACTOR
and
OBJECT
slots,
and
that
a
//LOCALE
should
f i l l the TO s l o t .
T h e n when t h e
w o r d s " J o h n " and "New
York"
build
such
structures,
those
expectations
d i c t a t e how t h e y a r e
i n c o r p o r a t e d i n t o t h e CD.
By
contrast,
the
CD
r e p r e s e n t a t i o n f o r t h e s e n t e n c e " J o h n h i e d t o New
Y o r k " , a s s u m i n g t h e unknown w o r d
"hied"
is
ignored, would simply be:
Kl:
(//PERSON FIRSTNAME
K2:
(//LOCALE STATE
(JOHN))
(*NY*))
T h i s r e p r e s e n t a t i o n i s s i m p l y a n u n o r d e r e d bag o f
noun
groups,
lacking
the s t r u c t u r e u s u a l l y i m p o s e d upon i t b y t h e v e r b .
Assuming
that
this
kind
of
structure
was
b u i l t in the event of a
verb f o u l - u p ,
the
later
stages
of
processing
would
be quite d i f f i c u l t .
T h i s u n s t r u c t u r e d bag
o f noun g r o u p s c a n n o t b e m a t c h e d i n t o
a
script,
at
least
not by the usual methods.
The FOUL-UP
program performs a f o u r - s t e p
process
to
enable
undefined
v e r b s to be c o r r e c t l y d e f i n e d in terms
of the context in which they appear.
b.
Step
1:
A " C a t c h - A l l " V e r b Frame
In the e v e n t t h a t
the
unknown
word
in
a
sentence
Is
a v e r b , FOUL-UP must c r e a t e a v e r b frame which i s capable o f e x p e c t i n g a l l the
subsequent
noun
groups
or other s l o t - f i l l e r s that
may o c c u r .
Such a " c a t c h - a l l " v e r b
frame
looks
l i k e the f o l l o w i n g :
Languafce-9:
175
nran*er
((ACTOR
(NIL)
<->
( N I L ) OBJECT ( N I L ) TO ( N I L )
FROM ( N I L ) ) TIME ( N I L ) )
Furthermore, each of the s l o t s in
this
CD-frame
has a s s o c i a t e d w i t h i t a s e t o f e x p e c t a t i o n s t h a t
w i l l accept v i r t u a l l y
any
conceptual
class
of
noun
that
may
appear.
This
ensures that a l l
words i n the sentence w i l l a t l e a s t
be
assigned
some p o s i t i o n i n t h e CD.
Note t h a t the r e s u l t i n g
CD, b u i l t b y E L I a n d FOUL-UP, w i l l
become
input
to
APPLY's
p a t t e r n - m a t e h e r , when i t a t t e m p t s t o
match the CD i n t o the s c r i p t .
Thus, it
is
cruc i a l t h a t the noun g r o u p s a r e put i n t o the c o r r e c t s l o t s w i t h i n the c o n c e p t u a l i z a t i o n ,
if
the
CD
is
to
be
recognized
correctly
by APPLY's
pattern-matcher.
Step
2:
|
P r e p o s i t i o n Matching
This step consists
of
using
knowledge
of
prepositions
to
tentatively
place
nouns
Into
their appropriate slots.
Thus
the
prepositions
TO,
TOWARDS, I N T O , A T e x p e c t t o f i l l t h e T O s l o t
i n t h e CD, w h i l e FROM, OFF, OUT
expect
to
f i l l
the
FROM
slot.
Finally,
if no preposition is
p r e s e n t , t h e OBJECT s l o t i s a s s u m e d .
This
heuristic
is dependable up to a p o i n t , but d i f f i c u l t i e s a r i s e when i d i o m a t i c
uses
of
prepositions
occur.
A s a n e x a m p l e , assume t h e s e c o n d s e n t e n c e
i n t h e a b o v e s t o r y had r e a d " T h e c a r caromed
off
a t r e e " , w h e r e " c a r o m e d " i s unknown t o SAM.
Then
the conceptualization for
"tree"
would
be
put
into
the
FROM
s l o t i n t h e CD, b y t h e h e u r i s t i c
j u s t s t a t e d f o r the p r e p o s i t i o n " o f f " .
However,
the
script
pattern
w i l l b e l o o k i n g f o r the obs t r u c t i o n " t r e e " i n t h e OBJECT
slot,
correspond i n g to the normal c o n s t r u c t i o n "The car s t r u c k a
tree".
Thus t h e p o s i t i o n o f t h e n o u n s i n t h e
CD
must
be
t h o u g h t o f a s t e n t a t i v e , a n d APPLY w i l l
o f t e n have t o s e a r c h more
than
one
conceptuali z a t i o n before it f i n d s the appropriate match.
d.
Step 3:
prepositions
occur
in
a
prepositional
phrase
combined
with
certain
conceptual
classes
of
n o u n s , t h e p r i m i t i v e ACT i n t h e s e n t e n c e c a n
often
be
predicted
simply from t h i s i n f o r m a t i o n .
The f o l l o w i n g t a b l e i s a s e g m e n t o f
a
table
of
such
correlations,
u s e d i n FOUL-UP t o h e l p p r e d i c t w h a t t h e ACT i n t h e s e n t e n c e w i l l b e .
Given
i n t h e t a b l e a r e common p r e p o s i t i o n s o n one a x i s ,
a n d common c o n c e p t u a l c l a s s e s o f n o u n s a l o n g
the
other
axis.
For
each
possible combination of
p r e p o s i t i o n and noun i n
a
prepositional
phrase
(e.g.
" t o " and "//PERSON" i n "He g a v e t h e b o o k _to
M a r y " ) , t h e c o r r e s p o n d i n g p r e f e r r e d ACT i s l o o k e d
up
in
the
table (e.g.
ATRANS), a s w e l l a s t h e
p r e d i c t e d s l o t in the CD f o r
the
noun
to
f i l l
(the TO s l o t ) .
ACT P r e f e r e n c e
The APPLY s e a r c h c a n
be
limited
to
those
c o n c e p t u a l i z a t i o n s I n the s c r i p t which c o n t a i n a t
l e a s t t h e same noun g r o u p s a s t h e
input
conceptualization,
even
i f t h e y ' r e not i n the c o r r e c t
slots.
T h e r e a r e s t i l l l i k e l y t o b e two o r
more
s u c h p o t e n t i a l m a t c h e s i n t h e s c r i p t , w h i c h APPLY
can n o t dependably d e c i d e between.
Thus
FOUL-UP
uses
an
a d d i t i o n a l h e u r i s t i c t o a i d the s e a r c h ,
t h a t o f "ACT P r e f e r e n c e " .
Based o n t h e
preposit i o n a l phrase t h a t appears i n the sentence, t h e r e
is quite o f t e n a unique
primitive
ACT
implied,
along
with
a
s e t of e x p e c t a t i o n s f o r the s l o t fillers.
T h u s i f w e see a
sentence
like
"John
hied
to
New Y o r k " , w e e a s i l y i n f e r , e v e n i n t h e
absence o f a s c r i p t o r o t h e r l a r g e c o n t e x t ,
that
t h e ACT i n t h e C D f o r t h i s s e n t e n c e w i l l p r o b a b l y
b e a PTRANS,
because
of
the
preposition
"to"
followed
by
a
location.
The
prepositional
phrases considered
are
those
which
contain
a
preposition
followed
by
a noun g r o u p .
The SAM
system is
able
to
classify
noun
groups
into
conceptual
classes,
such
a s #PHYSOBJ ( p h y s i c a l
o b j e c t ) , #PERSON (human b e i n g ) ,
#CONCEPT
(subclause),
#LOCALE
(place),
etc.
When
certain
Natural
OFF
//PHYSOBJ //PERSON //LOCALE //CONCEPT //BODYPART
PTRANS?
FROM
BY
| Script?
| INSTR
TO
|
PTRANS
FROM
MTRANS
MOBJECT
?
INSTR
PTRANS
LOC
?
INSTR
?
INSTR
ATRANS
TO
PTRANS
TO
PROPEL
TO
PTRANS
TO
MTRANS
MOBJECT
INGEST
TO
I
I N T O | PROPEL
I TO
For each o f these p a i r s o f
categories
(preposition
/
noun
c l a s s ) , sample sentences a r e abundant.
For example,
the
pair
(OFF
/
//LOCALE)
c o u l d a r i s e from the sentence "The man jumped o f f
t h e b u i l d i n g " , and the
pair
(INTO
/
//CONCEPT)
occur
in
the sentence "He looked i n t o the i n c i dent".
The i d e a o f " p r e f e r r i n g " a c e r t a i n s e t o f
attributes
when
in
the presence o f p a r t i a l i n f o r m a t i o n has been used
In
other
systems
(see
W i l k s , 1975).
Note t h a t the t a b l e i s i n c o m p l e t e ,
and thus can n o t always p r o v i d e
enough
informat i o n t o a l l o w the c o r r e c t c o n c e p t u a l i z a t i o n t o b e
chosen from the s c r i p t .
Further
work
is
being
done i n t h i s a r e a .
e.
Step 4:
Matching
R e c a l l the s t o r y t h a t
began
this
section.
Based
on
the
above t a b l e , t h e second s e n t e n c e ,
"The car hied i n t o a t r e e " , would be
represented
as f o l l o w s :
Kl:
((ACTOR (K2) <-> (BOGUS-ACT LEXVAL (HIED)
PREFER (PROPEL)) TO (K3))
K2:
(//PHYSOBJ TYPE
(*CAR*))
K3:
(#PHYSOBJ TYPE
(*TREE*))
Note t h a t t h e ACT "PROPEL" is p r e f e r r e d , and
the
conceptualization
for
the noun " t r e e " i s put i n
t h e "TO" s l o t .
When APPLY a t t e m p t s to match t h i s
conceptualization
into
the s c r i p t , i t d i s c o v e r s
one t e n t a t i v e m a t c h ,
w h i c h has
the
same
noun
conceptualizations,
corresponding
to
" c a r " and
" t r e e " , but the one f o r " t r e e " i s i n
the
OBJECT
slot
i n the s c r i p t c o n c e p t u a l i z a t i o n , and i n the
Language-9:
176
Granger
T O s l o t i n t h e CD, s o t h e m a t c h c a n o n l y b e
considered
tentative.
(This
i s a n example o f t h e
i d i o m a t i c usage o f
verb-preposition
pairs
mentioned
e a r l i e r . ) APPLY now l o o k s a t t h e i n f o r m a tion
in
the
PREFER
slot
of
the
"BOGUS-ACT"
p l a c e - h o l d e r , w h i c h s a y s t h a t t h e ACT s h o u l d b e a
PROPEL.
Since t h i s is in f a c t
the
ACT
in
the
s c r i p t c o n c e p t u a l i z a t i o n , the match i s c o n s i d e r e d
s u c c e s s f u l , and t h e word
"hied"
is
defined
as
building
a
PROPEL i n t h i s c o n t e x t ,
The d e f i n i t i o n t h a t FOUL-UP c r e a t e s f o r " h i e d "
looks
like
the f o l l o w i n g :
(PROG N I L
(DEFPROP HIED T VERB)
(1)
(DEFPROP HIED *CONCEPT* I S A )
(DC HIED
(2)
(T ( N I L (BOGACTFRAME) N I L
ACTORSUGG BYSUGG TOSUGG
(OBJECT CONREQ) TIMESUGG MODESUGG
(FROM CONREQ) (TO CONREQ))
( N I L (THREEFRAME (QUOTE H I E D ) ) N I L
(3)
SUBJSUGG (OBJ SYNREQ) RECIPSUGG)))
Again, the d e f i n i t i o n i s i n
three
parts:
part
(1)
puts
the
appropriate
internal
properties
" v e r b " and " c o n c e p t " o n t h e w o r d ;
part
(2)
defines
the
conceptual
(semantic)
structureb u i l d e r s and t h e e x p e c t a t i o n s f o r t h e
subsequent
slot-fillers;
and ( 3 ) d e f i n e s t h e s y n t a c t i c e x p e c t a t i o n s and
slot-fillers
(see
Riesbeck
and
Schank,
1976).
Note
that
a l l the i n f o r m a t i o n
provided by the f i r s t
three
steps
(verb-frame,
p r e p o s i t i o n m a t c h and ACT p r e f e r e n c e ) was i n f a c t
r e q u i r e d by the matching process b e f o r e the
conceptualization
could
be recognized as a form of
the p a t t e r n i n the s c r i p t .
f.
N o t e s o n how h a r d
this a l l
is
The p r o c e s s j u s t d e s c r i b e d
depends
on
the
current
state
of
knowledge o f (1) p r e p o s i t i o n s
and ( 2 ) c o n c e p t u a l c l a s s e s o f n o u n s .
The
theor i e s b e h i n d each o f these a r e q u i t e p r i m i t i v e (no
pun i n t e n d e d ) , and t h e a b o v e p r o c e s s f o r f o u l - u p s
of
verbs
Is at a correspondingly e a r l y stage of
development.
The p r o b l e m o f r e s o l v i n g
the
case
ambiguity
of
text
prepositions
has p r o v e d i n tractable
in
many
natural
language
systems.
General
rules
for prepositions are riddled with
e x c e p t i o n s and s p e c i a l c a s e s , and e v e r y
preposition
seems t o h a v e a m y r i a d d i f f e r e n t u s e s .
For
an example of t h e d i v e r s i t y of uses of
a
single
preposition
( " f o r " ) , see ( H e m p h i l l , 1 9 7 3 ) .
Simi l a r l y , the theory behind conceptual
classes
in
SAM
is
c o n s i d e r a b l y less developed than that of
p r i m i t i v e ACTs.
Other sets of conceptual classes
and
object
p r i m i t i v e s have been proposed in the
p a s t , and p e r h a p s t h e SAM
system
(and
FOUL-UP)
would
benefit
from a r e - e x a m i n a t i o n of the c u r rent
classification
system
for
nouns.
(See
W e b e r , 1 9 7 1 , and L e h n e r t , 1 9 7 7 ) .
4.
Adjectives
Consider
the
story:
Natural
" F r i d a y , a car swerved o f f
flibby vehicle struck a tree."
Route
69.
The
The m e a n i n g o f t h e w o r d " f l i b b y "
in
the
second
sentence
is
not
at
all
clear,
nor can I t b e
e a s i l y f i g u r e d out from the c o n t e x t .
In fact, it
is
not
clear
whether o r n o t the a d j e c t i v e even
changes the meaning o f the sentence a t a l l , s i n c e
the
sentence
can
b e u n d e r s t o o d i n t h i s case b y
i g n o r i n g t h e unknown w o r d .
I n f a c t , FOUL-UP
attempts
to
ignore
unknown
adjectives
wherever
p o s s i b l e , and c a n n o t g u e s s m e a n i n g s f o r
them
in
general.
The
methods
used f o r nouns and v e r b s
t o t a l l y b r e a k down when
applied
to
adjectives.
There are three c l o s e l y r e l a t e d reasons f o r t h i s :
(1) A d j e c t i v e s are not expected in a sentence, in
the
sense
of
expectation
we have been d e a l i n g
with.
Rather an a d j e c t i v e appears as a m o d i f i e r ,
a f f e c t i n g t h e m e a n i n g o f t h e noun f o l l o w i n g i t .
(2) There
exist
no
primitives
for
adjectives
corresponding
to
the
relatively
comprehensive
s e t s o f p r i m i t i v e s w i t h i n t h e r e a l m s o f n o u n s and
v e r b s in CD r e p r e s e n t a t i o n .
( 3 ) The c o n c e p t u a l i z a t i o n b u i l t b y
an
adjective
usually
occupies
an
"extra"
s l o t i n a CD, a t t a c h e d t o t h e s l o t f i l l e d b y t h e noun
which
the
adjective
modifies.
Thus
there
is
no purely
" t o p - d o w n " way t o g u e s s a m e a n i n g f o r a n
unknown
a d j e c t i v e , since i t i s i n h e r e n t l y unexpected.
Note
t hat
the
ideas
of
expectation,
conceptual
classes,
and
slot-filling
are
all
c l o s e l y r e l at e d .
The e x p e c t a t i o n s
in
both
ELI
and
APPLY a r e
based o n d i f f e r e n t i a t i o n between
v a r i o u s cone e p t u a l c l a s s e s o f
objects,
and
the
slots
are f i l l e d
on
the b a s i s of the e x p e c t a tions.
Thus t h e l a c k o f
conceptual
classes
of
adjectives
causes
the
other
d i f f i c u l t i e s , and
they should not be viewed
as
independent
problems.
CONCLUSION:
*•
Syntactic
L I M I T I N G CASES
Class L i m i t a t i o n s
I t was s t a t e d e a r l i e r t h a t t h e r e m i g h t b e n o
theoretical
limitation
on
the c l a s s e s o f words
t h a t FOUL-UP c o u l d f i g u r e o u t
from
context,
as
long
a s t h e r e e x i s t e d E L I and APPLY e x p e c t a t i o n s
f o r the c o n c e p t u a l i z a t i o n s b u i l t by a
word.
We
have
now s e e n , h o w e v e r , t h a t d i f f e r e n t s y n t a c t i c
c l a s s e s o f words p r e s e n t v a r y i n g degrees o f
diff i c u l t y t o t h e FOUL-UP p r o g r a m .
The d i f f i c u l t i e s
a r i s e depending on (1) the c o n s i s t e n c y
and
completeness
of
the
existing
representational
schemes f o r a g i v e n s y n t a c t i c c l a s s , and ( 2 )
the
amount o f i n f o r m a t i o n c o n t a i n e d i n t h e s t r u c t u r e s
t y p i c a l l y b u i l t b y words i n t h a t s y n t a c t i c c l a s s .
Furthermore,
w e have seen t h a t the i s s u e s o f e x p e c t a t i o n o n t h e one h a n d , and
conceptual
class
on
the
o t h e r , cannot be e a s i l y separated.
Thus
t h e e x p e c t a t i o n s f o r t h e members o f c e r t a i n
syntactic
classes are s t r o n g e r than f o r o t h e r s , depending on the r e l a t i v e thoroughness of the
representation
schemes
for
those
classes.
Thus
strength of expectation
is
also
a
(dependent)
factor
c o n t r i b u t i n g t o t h e ease o r d i f f i c u l t y o f
t h e FOUL-UP t a s k w i t h i n a g i v e n s y n t a c t i c c l a s s .
Language-9: Granger
177
Nouns a r e t y p i c a l l y
slot-fillers,
builders
of
small s t r u c t u r e s containing r e l a t i v e l y l i t t l e
o f the o v e r a l l i n f o r m a t i o n
present
in
a
given
conceptualization.
They
a l s o have a reasonably
c o n s i s t e n t and complete r e p r e s e n t a t i o n
in
terms
of
conceptual
classes.
Thus
the
process f o r
f i g u r i n g out unknown nouns from c o n t e x t i s
relatively
straightforward.
Verbs
are b u i l d e r s o f
l a r g e s t r u c t u r e s which c o n t a i n most
of
the
exp e c t a t i o n s f o r a g i v e n s e n t e n c e , and which supply
most
of
the
structure
to
conceptualizations.
They
also
have a c o n s i s t e n t and complete r e p r e s e n t a t i o n in terms of p r i m i t i v e ACTs.
Thus
they
are
more
difficult
to
f i g u r e out from c o n t e x t
than nouns.
A d j e c t i v e s are not w e l l
defined
or
consistently
represented
in
CD, and they b u i l d
s t r u c t u r e s o f v a r y i n g s i z e and c o m p l e x i t y .
Furthermore,
being
conceptual m o d i f i e r s , they l a c k
pre-defined slots to f i l l in a conceptualization.
Thus
i t i s n a t u r a l t h a t they s h o u l d b e most d i f f i c u l t , i f not i m p o s s i b l e ,
to
figure
out
from
context.
2.
Conceptual Class L i m i t a t i o n s
Assuming t h a t a l l words can e v e n t u a l l y
have
their
meanings
represented in something a k i n to
C o n c e p t u a l Dependency r e p r e s e n t a t i o n , t h e n
there
would
still
b e l i m i t a t i o n s o n FOUL-UP's a b i l i t y
to f i g u r e out words f r o m c o n t e x t .
To p e r f o r m the
process,
a
strong
top-down
context,
like
a
s c r i p t , i s needed t o
provide
the
expected
attributes
o f unknown w o r d s .
Other l a r g e top-down
f r a m e w o r k s , s u c h a s p l a n s ( s e e Schank &
Abelson,
1977)
or
belief
systems
(see
Abelson, 1973),
should t h e o r e t i c a l l y provide
enough
context
to
enable
words
to
be f i g u r e d out from those c o n texts.
For
example,
consider
the
following
"planny" story:
" J o h n saw a m e n a c i n g f i g u r e a p p r o a c h i n g
his
store.
He
reached
into
the
d e s k d r a w e r and
p u l l e d o u t a magnum."
The w o r d "magnum" i n t h i s
story
is
highly
expected
to
be
some k i n d o f weapon o r o t h e r p r o tective device.
That e x p e c t a t i o n could be set up
by
a p l a n - a p p l i e r m e c h a n i s m (PAM) ( s e e W i l e n s k y ,
1 9 7 6 ) , and a v e r s i o n o f FOUL-UP
could
theoretically
use s u c h e x p e c t a t i o n s t o g u e s s t h e m e a n i n g
o f t h e unknown w o r d .
However, s c r i p t s
are
very
explicitly
defined
sequences
of actions, while
p l a n s a r e much
less
explicitly
sequenced,
and
belief
systems
still
less so.
It is to be expected t h a t the a b i l i t y t o f i g u r e out words
from
context
will
diminish
a s the s t r i c t l y e p i s o d i c
n a t u r e (and thus t h e
strength
of
the
expectations)
of
the
knowledge
database
diminishes.
Thus s c r i p t y s t o r i e s a r e i d e a l l y c o n s t r u c t e d
for
figuring
out
words from c o n t e x t , p l a n s a r e l e s s
s o , and b e l i e f s y s t e m s s t i l l l e s s .
It is
worthw h i l e t o r e i t e r a t e t h a t the r e p r e s e n t a t i o n s being
discussed are intended as
models
of
the
human
understanding
process,
and t h u s t h a t t h e a b i l i t i e s and l i m i t a t i o n s o f t h e p r o g r a m s a r e m o d e l l e d
a f t e r analogous processes in p e o p l e .
The FOUL-UP
p r o g r a m was d e s i g n e d to s i m u l a t e
a
known
human
ability,
and
t o add t h i s a b i l i t y t o t h e a l r e a d y
Natural
e x i s t i n g SAM s y s t e m .
S i n c e SAM
is
intended
to
model
t h e human u n d e r s t a n d i n g p r o c e s s w i t h i n t h e
realm of script-based s t o r i e s , it is
encouraging
to
n o t e t h a t t h e l i m i t a t i o n s o f t h e FOUL-UP seem
t o c l o s e l y p a r a l l e l the l i m i t a t i o n s
of
people's
ability
to
perform
t h e same t a s k .
Certainly a
p r o g r a m i n t e n d e d t o m o d e l humans w i l l n o t b e a b l e
to
out-perform
t h e m , b u t r a t h e r s h o u l d show t h e
same s o r t s o f
abilities
and
shortcomings
that
people
show.
I n t h a t s e n s e , t h e FOUL-UP p r o g r a m
h a s d e m o n s t r a t e d some p s y c h o l o g i c a l v a l i d i t y
for
the
SAM
system,
and
f o r C o n c e p t u a l Dependency
representation I t s e l f .
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H.
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178
Grander