STRUCTURE OF SENTENCE AND INFERENCING IN QUESTION ANSWERING 
Eva Haji~ovA and Petr Sgall 
Faculty of Mathematics and PhTeics 
Charles University 
: '. '. ~.. " • Malostranak@ n. 25 
118 O0 Praha 1 
Czechoslovakia 
ABSTRACT 
In the present paper we characterize 
in more detail some of the aspects of a 
question answering system using as its 
starting point the underlying structure of 
sentences (which with some approaches can 
be identified with the level of meaning 
or of logical form). First of all, the 
criteria are described that are used to 
identify the elementary units of under- ~ 
ing structure and the operations con- 
oining them into complex units (Sect.l), 
then the main types of ~n~ts and operations 
resulting from an empirical investigation 
on the basis of the criteria are register- 
ed (Sect.2), and finally the rules of in- 
ference, accounting for the relevant 
aspects of the relationship between ling- 
uistic and cognitive structures are 
illustrated ~Secto3). 
I. A system of natural language 
understanding may gain an advantage from 
using the underlying structure of sent- 
ences (which with some approaches can be 
identified with the level of meaning or 
of logical form) as one of its starting 
points, instead of working with word 
specific roles. Ar~menta for such a 
standpoint, which were presented in Haji- 
~ov~ and S~all (1980), include the follow- 
ing two maln points: 
(a) natural language is universal, 
i.e. its structure makes it possible to 
express an unlimited n~-.ber of assertions, 
questions, etc° t by finite means} once 
its underlying (tectogrammatical) struct= 
ure is known, it is possible to use it ai 
an output language of natural language 
analysis in man-machine communication and 
thus, without any intellectual effort on 
the side of the user, to ensure the funct- 
ioning of automatic question answering 
systems (or of systems of dialogues with 
robots, etc.)} even if many simplificat- 
ions have been included into such a 
system, it is then known what has been 
simplified and it is possible to remove 
the simplifications whenever necessary 
(e.g. if the system is to be used for an- 
other set of tasks, including the anal- 
ysis of a broader set of input texts, 
questions, etc.); 
(b) linguistic meaning is ~ystem- 
atic, so that the configurations of 
"deep cases" (valency), tenses~ modalito 
ias, number, etc. make it possible to 
find full~ reliable information; on the 
other hand, such systems as those baaed 
on scenarios or scripts work in most 
cases with rules that are valid for the 
unmarked cases (in a marked case e.g. 
lunch in a restaurant can be taken by an 
employee of the restaurant, who does not 
reserve a table, order the meals and P~7 
for them ***)° 
To find out which of the semantic 
and pragmatic distinctions are reflected 
in the system of language ~or, in other 
words, to find out in what respects the 
underlying structure of sentences differ 
from their surface patterns) testable 
operational criteria are needed~ these 
criteria should help to distinguishl 
(i) whether two given surface -_nits 
are strictly synonymous (i°e. share at 
least one of their meanings), or not~ 
(ii) whether a single surface unit 
has more than one meaning (is ambiguous), 
or whether a sibgle meaning is concerned s 
which is vague or indistinct (cf. Zwicky 
and Sadock, 1975; Kasher and Gabbay, |976} 
Keenan, 1978); 
(iii) whether a given distribution- 
al restriction belongs to the tectogra--.- 
atical level, or whether it is given 
onl~ by the cognitive content itself, i.e. 
by extralinguistic conditions; 
(iv) between a case of deletion (of 
a tectogra~saatical unit by surface rules) 
and the absence of the given unit in the 
underlying structure; 
(v) between different kinds of 
tectogrammatical units (e.g. inner part- 
icipants of cases, and free or adverbial 
modifications); 
(vi) which tectogrammatical unit 
has been deleted, in case more of them 
can occupy the deleted position (el. 
21 
the tectogrammAtical difference between 
the elements of the topic and those of 
the focus of the sentence, or more exactly, 
between contextually bound and non-bound 
elements of the meaning of the sentence). 
As for (i), a criterion has been 
elaborated that works similarly as Car- 
nap s intensional isomorphism, but is 
adapted for the structure of natural lan- 
guage, the surface gr-mmAtical means of 
which also exhibit synor%vmY: He expected 
that Mary comes and He expectedMary to 
come are considered synonymous, since 
wl---~any lexical (and morphological) 
cast such two sentences correspond to 
a single proposition (a single truth 
value is assigned to any possible world). 
On the other hand .John talked to 
a girl about a problem is not considered 
to be synonymous with John talked about 
a problem to a girl, since the known 
(Lakoff s) examples with a specific ~ 
uantification do not share their truth 
onditions; also our simple examples 
differ in their tectogr~mmatical struc- 
tures (having different topic-focus ar- 
ticulations). 
For points (ii), (iii) and (v) the 
classical criteria known from European 
structural linguisti~ are used, such as 
the diagnostic contexts~ possibility of 
coordination, or Keenan s (1978) criter- 
ion of the necessary knowledge of the 
speaker whether s/he uses an ambiguous 
item in this or that of its meanings. 
It should be noted that perhaps each of 
the criteria has its weak points (often 
the implications work in one direction 
only, xn some cases not only surface fea- 
tures, but also the tectogrammatical cha- 
racter of the context has to be taken in- 
to account, etc.). 
Point (iv) can be systematically 
tested by means of the so-called dialogue 
test (cf. Haji~ov~ and Panevov~, in press): 
e.g. in John came the direction (rather 
than the~ point or the time point) 
has been deleted, so that the speaker 
necessarily knows where John came and can 
answer such a question (though s/he may 
not know from where of when John came). 
With respect to point (vi) the 
question test or the tests concerning 
negation can be used~ as far as the topic- 
-focus articulation is concerned; thus 
e.g. in John sent a letter to his SISTER 
the verb as well as the Objective are 
ambiguous, since the sentence can (in 
different contexts) answer e.g. such 
questions as What did John do? (only John 
being include~'in the topic of the answer, 
all the rest belonging to its focus), 
W~a% did John send where? (also the verb 
belonging to the topic of the answe@ 
What did John do with the letters? (a 
letter rather than the verb being included 
in the topic), etc.; the criterion shows 
that John belongs to the topic in all 
readin-g~-of the sentence (since John is 
contained in all relevant question, if 
such improbable or secondary pairs are 
excluded as our sentence answering the 
questien What happened?without John re- 
ferring to one of the most activ--~d ele- 
ments of the stock of shared knowledge at 
the given time point), and that his sister 
belongs to the focus (not occurring in 
any relevant question). 
2. The framework resulting from an 
application of the criteria characterized 
in Sect. I can be briefly outlined as 
follows: 
The elementary units of the under- 
lying structure are of three kinds: 
(a) lexical elements (semantic featu- 
res); in the present paper we do not deal 
with operations or relations concerning 
the combining of features into more or 
less complex lexical meanings; 
(b) elementary gramatical meanings 
(grammatemes), which can be classified 
as values belonging to various catego- 
ries or parameters (delimitation, number, 
tense, aspect, different kinds of moda- 
lities, etc.); 
(c) syntactic elements (functors) 
such as Actor, Addressee, Instrument, 
Directional, etc. 
The underlyin~ structure of a sen- 
tence can be concexved of as a network 
(which can be linearized, see Pl~tek, 
Sgall and Sgall, in press) the nodes 
and edges of which are labelled. A label 
of a node consists of a lexical meaning 
and a combination of ~rammatemes from 
different categories (the set of relevant 
categories is determined by the word class 
of the lexical meaning). A label of an 
edge consists in a functor, which is in- 
terpreted either as a Dependency relation, 
or as one o~ the relations of Coordinati- 
on (corresponding to the meanings of and, 
or~ but, etc.) or of Apposition. The ~- 
pendency re--6Iations are combined (in the 
underlying structure of a sentence 
without coordination or apposition) into 
a projective rooted tree, the nodes of 
which are ordered (from left to right) 
according to the scale of communicative 
dynamism, which is decisive for the to- 
pic-focus articulation of the sentence. 
The relations of Apposition anS Coordina- 
tion are combined with those of Depend- 
ency according to certain rules described 
in the last quoted paper and illustrated 
by Fig. 1 to 3. 
22 
BE 
Act~bj 
AMPLIFIER DEVICE 
OPERATIONAL 
Gener~ _ _ 
Act 
eoeeee~oeeeeeeee APPLY-Inter CONDITION 
Ac~bj Act~~bj 
DGEN DEVICE DGEN SIGNAL 
Figure 1. 
A simplified underlying representation of Operational amplifier is a versatile device 
with applications spanning signal conditioning and special s~stems design; Gemer is 
the functor of general relation (the kind of dependency often found between a noun 
and its modifications), the other symbols are self-explanatory; the grammatemes are 
written only if they are marked, i.e~ Present, Indicative, Singular, Specifying are 
understood as determined by default . 
DESIGN 
Ac~Obj 
DGEN SYSTEM.PIu~ ~ ~ 
ner 
SPECIAL 
Or 
• ° • • • • • • • • • • • • • • • 
• . • • , • • • ° • • ° 
• • o 
• ° VISIT 
. • • And VISIT And VISIT 
JANE MARY TOM JANE FAMILY JANE MOTHER 
&ppurt 
WE 
• • . . • ° • 
JANE HOME 
, Figure 2. 
A simplified underlying representation of Jane either visits Mar~ and Tom t our famil~! and Mothert, or she sta~s at home. 
23 
LIVE 
Act Loc 
MARY JOHN 
O~ner 
FXND-Pret 
MARY JOHN FAMILY 
BOSTON 
Figure 3. 
A simplified underlying representation 
of Mar~ and John, who founded a family , 
live in Boston. 
Fig. i points out how phrasal coordination 
is handled; in Fig. 2 a configuration of 
two sentence coordinations (wxth dele- 
tions) appears; Fig. 3 illustrates cases 
where two coordinated nodes have an ex- 
pansion (relative clause) in common. 
If interjectional sentences, vocati- 
ve sentences and pseudosentences consis- 
ting onl~ in a noun phrase ere not discu- 
ssed, then it can be stated that the root 
of every tree of the mentioned kind is 
labelled by a symbol the lexical part of 
which belongs to the word class of verbs. 
The kinds (and to a certain part also the 
order) of the dependency edges going from 
a node to those dependent on it are de- 
termined by the valency frame of the go- 
verning word (included in the lexical 
entry of the given lexical meaning). The 
kind of dependency relation are specified 
in two respects,which are relevant for 
their combinatorial properties: (a) they 
are classed either as (inner) participants, 
namely Actor (i.e. Actor/Bearer, or Tesni- 
~re °% premier actant rather than Fill- 
more s Agentive), Objective, Addressee, 
Origin and Effect, or as (free) modifica- 
tions, i.e. Instrument, Manner, Locative, 
several kinds of Directional and Temporal 
modifications, Cause, Condition (real and 
irreal), etc.; (b) they are either obli- 
atory, or optional. Every participant 
hich occurs only with some governing 
words, and at most once as dependent on 
the same token of the governing word) is 
included in the valency frames of all 
words on which it can depend; the free 
modifications are the same for all words 
belonging to the same word class (on the 
level of underlying structures), so that 
they can be listed once for all; only 
those modifications that are obligatory 
with a given lexical unit are quoted in 
its frame. 
Two specific cases are important for 
the empirical investigations: (i) a depe- 
ndent word present in the underlying stru- 
cture but deleted in the surface should 
be distinguished from the absence of the ~ 
iven element on the underlying structure; 
ii) with the inner participants it is 
also necessary to distinguish between the 
absence of an (optional) participant and 
a general participant of the fiven kind 
(this does not concern only the general 
Actor, typicall~ expressed by ~ne in Eng- 
lish, but also the Objective, c-~. Haji~o- 
v~ and Panevov~, in press). 
3. With this approach, the underlying 
structures are relatively close to the 
surface structure of sentences. This is 
connected with the advantages granted by 
the universal character of natural langu- 
age (ensuring that the framework is nottoo 
narrow and can be generalized if applied 
to a larger class of texts, etc.). On the 
other hand t with such a framework it is 
necessary to use a model of natural langu- 
age inferencing, if we want the procedure 
of language understanding to go beyond pu- 
rely linguistic relationships. If e.g. in 
a question-answering system based on such 
a framework not only such answers should 
be identified that were literally present 
in the input text, but also those yielded 
by simple (mostly unconscious) inferenc- 
ing normally carried out by the reader of 
the text, then rules of inference can be 
added. A first tentative set of such rules 
is being checked in the experiments with 
the system prepared on the basis of the 
method TIBAQ in Prague. These rules range 
from general ones to more or less idio- 
syncratic cases concerning the relation- 
ships between specific words, as well as 
modalities, hypo~ym~, etc. 
24 
A rather general rule changes e. ~. a 
structure of the form (V-act(NAct.r).g.) 
into (V-act(DActor(Nlnstr)...) , v 
where V-act is a verb of action, D is a 
dummy (for the general actor) and N is an 
inanimate noun; thus The negative feed- 
ba6k can servo the volta6e to zero is 
changed into One can serve the voltage to 
zero by .... A rather specific rule 
connected with a single verb is that chan- 
ging (use (Spatien t) (XAccomp) .... into 
(use (X- ~) (Y .... ) ...), e.g. An 
op--e~ati~r~pli~e~n be used wit~-a 
negative feedback = With an operational 
ut~lifier a negative feedback can be used. 
er similar rules concern t~e division 
of conjunct clauses, the possible omissi- 
on of an adjunct under certain conditions 
(i.e. if not being included in the topic, 
e.g. from "It is possible to maintain X 
without emplcying Y" it follows that it 
is possible to maintain X), or several 
shifts of verbal modalities, asp. a sen- 
tence having the main verb with a Possi- 
bilitive modality (can, may) is derived 
from a positive deca~tlv~-'6 sentence; in 
some cases (when the name of a device 
occupies the posit~n of the Actor of the 
main verb) also a reverse rule is avai- 
lable, deriving e.g. The device X is used 
with a ne6ative feedback from The device 
X can be used with anegative feedback. 
l~urther rules yield a conjunction or a 
similar connection of two statements; 
e.g.X is a device with the property Y 
and X can be applied to handle Z are 
combined to yield X is a device %hat has 
the property Y and can be applied to han- 
~ also explicit definitions (inclu- 
.g. the verb call) are identified 
and the inference ru--~ allow for repla- 
cements of the definiendum by the defini- 
ens and vice versa in other assertions, 
Besides these kinds of rules it is 
necessary to study (i) rules standing 
closer %o inference as known from logic 
(deriving specific statements from general 
ones, etc.), (ii) rules of "typical" (un- 
marked) consequence as given e.g. by a sc- 
ript~ and (iii) rules of "probable conse- 
quences", e.g. if John worked hard in the 
afternoon and he is tired in the evening, 
then the latter fact probably was caused 
by the former ~if no other cause was gi- 
ven in text). In our experiment of ques- 
tion answering we do not use these types 
of inference, but they will be useful for 
more general systemS. 
Another direction in which the system 
probably can be made more flexible concerns 
the absence of overt quantifiers and mar- 
king of their scopes in our underlying 
structures. One of our next aims consists 
in the construction of a procedure trans- 
ducing the underlying structures into a 
mixed language, which would include means 
for marking quantifiers and their scopes 
(similarly to many formal languages of lo- 
gic), while it would share all other as- 
pects of its structure with the level of 
unle~lying representations of natural lan- 
guage. 
Colmerauer's Q language is used for 
the implementations of the main procedu- 
res of the question-answering system, so 
that e.g. A(B,C(D,E)) represents a tree 
the head of which is ~, which has two 
sister nodes, B, C, the latter being again 
expanded by D and E. The tree structure 
is used in our syntactico-semantic analy- 
sis of Czech (prepared by J.Panevov~ and 
K.Oliva) and of English (by Z.Kirschner) 
to represent the dependency relation 
between nodes. Due to the fact that Q lan- 
guage works only with elementary labels, 
the complex labels of our description have 
to be decomposed (i.e.the features and 
grammAtemes of individual work forms occu- 
py similar positions as their daughter 
nodes). Also the procedures for the app- 
lication of inference rules and for the 
identification of (full and partial or 
indirect) answers to a question given by 
the user (on the basis of the corpus of 
input texts that have been analyzed) are 
programmed in Q language. The synthesis 
of Czech and morphemic analysis are im- 
plemented in PL/I. For a more general sys- 
tem the set of inference rules should be 
substantially enlarged, and various heur- 
istics, strategies and filters should be 
formulated in order to keep the number of 
derived assertions in fixed limits. For 
these aims the experience gained in the 
first experiment will be used. 
REFERENCES 
Haji~ov6 E. and J° Panevov~ (in press), 
Valency (Case) Frames of Verbs, in 
Lualsdorff and Sgall (eds°) 
Ha~i~ov~ E° and P. Sgall (1980), Linguistic 
Meaning and Knowledge Representation 
in Automatic Understanding of Natural 
Language, Prague Bull. of MathematiC- 
al Linguistics 34, 5-19 
Kasher A. and De-M° Gabbay (1976), On the 
Semantics and Pragmatics of Specific 
and Non-Specific Indefinite Express- 
ions, Theoretical Linguistics 3,145ff. 
Keenan Ee (1978), Some Logical Problems in 
Translation, in Meaning and Translat- 
ion (ed. by F° Guenthner and M. Guenth- 
ner-Reutter), London, 157-189 
Lualsdorff P. and P. Sgali ~eds.~, Contrib- 
utions to Functional Syntax~ Semant- 
ics and Language Comprehenslonp to be 
published by Ben,amine and Academia 
Pl~tek Mo, Sgall J. and Po Sgall (in press), 
A Dependency Base for a Linguistic 
Description, in Luelsdorff and SgalI 
(eds.) 
25 
