DEALING WITH THE NOTION "OBLIGATORY" IN SYNTACTIC ANALYSIS 
Dorothee Reimann 
Zentralinstitut f~r Sprachwissenschaft 
Akademie der Wissenschaften der DDR 
Prenzlauer Promenade 149-152 
Berlin 
DDR - II00 
ABSTRACT 
In the paper the use of the notion 
"obligatory complement" in syntactic 
analysis is discussed. In many theories 
which serve as bases for syntactic 
analysis procedures there are devices to 
express the difference between obligatory 
and optional complements on the rule 
level, i.e. via the lexicon the wordforms 
are connected with these rules where the 
fitting properties are expressed. I'll 
show that such an approach leads to some 
problems, if we want to handle real texts 
in syntactic analysis. 
In the first part I'll outline the 
theoretical framework we work with. Then 
I'll discuss for which purpose the use of 
the notion obligatory has some advantages 
and in the last part I'll show shortly how 
we intend to use this notion 
- in lexical entries (with respect to 
morphological analysis) and 
- in the syntactic analysis process. 
SOME THEORETICAL PREREQUISITES 
The basis of our work is a special 
version of a dependency grammar (Kunze 
1975). In this theory a syntactic 
structure of a sentence is represented as 
a tree, where the nodes correspond to the 
wordforms of the sentence and the edges 
express the dependencies between the word- 
forms. The edges are marked by subordina- 
tion relations (SR's) which describe the 
relation between the subtree "under" the 
edge and the remaining tree context. 
Besides the syntactic dependencies 
other connections between the wordforms of 
the sentence remain which express certain 
congruences and restrictions. Here we have 
congruences - so-called paradigmatic 
connections - like (the listed categories 
concern the German variant): 
from a noun to an attribute (gender, 
number, case) 
from a preposition to the noun (case) 
from the subject to the finite verb 
(number, person) 
and restrictions - selective connections - 
like: 
from the verb to the (deep) subject 
from the verb to the direct object etc. 
The selective connections also apply to 
all transformational variants of the 
concerned phenomenon (let us take the 
SUBJ-connection): 
(I) 
(2) 
(3) 
(4) 
(5) 
(6) 
John liest ein Buch. 
(John reads a book. ) 
Ich sehe John ein Buch lesen. 
(I see John reading a book. ) 
Das Buch wird von John gelesen. 
(The book is read by John.) 
Das von John gelesene Buch ... 
(The book read by John ...) 
Das Lesen des Buches durch John .. 
(The reading of the book by John .) 
Der ein Buch lesende John ... 
(John reading a book ...) 
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(7) John, der ein Buch liest .... 
(John who reads a book .... ) 
It is easy to see that the tree 
property would be destroyed if these 
connections were included as edges in the 
tree. To save the tree property Kunze 
introduced the mechanism of paths of 
action for the paradigmatic and selective 
connections. These paths run along the 
edges, i.e. they can be expressed also by 
the subordination relations. This is one 
essential reason for differentiating the 
SR's very strongly. 
For instance, it is necessary to 
differentiate between 
- the "normal" direct object and the 
direct object with subject role: 
John reads a boo~. 
I see Joh~ reading a book. 
- an adjective as attribute and a 
participle as attribute: 
The ~E book ... 
The r_~gding John ... 
- the subject in an active clause and the 
subject in a passive clause: 
John reads a book. 
A boQ~ is read by John. 
Besides the subordination relations 
another central concept in Kunze's theory 
are the h~les (see also Reimann, 1982). 
A bundle is a substructure of a dependency 
tree which contains exactly one top node 
and all nodes directly subordinated to it 
together with the edges between (and their 
markings - the subordination relations). 
The original idea was to use the bundles 
as syntactic rules. For this purpose, the 
bundle is regarded as a system of 
conditions which have to be fulfilled by a 
set of nodes to construct the structure 
which the bundle prescribed. 
But another possibility to use bundles 
is the following: They can serve as 
descriptions for the dominance behaviour 
of wordforms (i.e. the surface form of 
valency). In this way, the approach is 
similar to other theories: In the lexical 
entries of the wordforms there is a 
pointer to the rules which can be applied 
with the concerned wordform as top node. 
Our approach goes farther in the 
direction of dominance behaviour 
descriptions. Having in mind that, 
especially for nouns and verbs, the 
dominance behaviour is a very complex one, 
i.e. many different things can be sub- 
ordinated to nouns and verbs: many of them 
are optional, some of them stand in 
certain relations to others, etc. Thus we 
concentrate all these bundles by defining 
another form of a bundle, which consists, 
in general, of many simple bundles. 
For instance: 
Peter stiehlt. 
Peter stiehlt ein Auto. 
Peter stiehlt dam Bauern das Auto. 
Peter stiehlt dam Bauern das Auto vom 
Hof. 
Peter stiehlt das Auto vom Hof. 
* Peter stiehlt vom Hof. 
* Peter stiehlt dam Bauern. 
As we can see, only the subject is 
obligatory (in the active sentence), but 
the indirect object as well as the 
directional circumstance are only used, if 
the direct object belongs to the sentence. 
These facts can be expressed by a logical 
formula like this: 
(SUBJ @a v ((IOBJ vDIR)-~DOBJ)) 
That means we represent the dominance 
behavicur of wordforms by logical formulas 
(in subordination relations) - we call 
these formulas bundles. It is quite clear 
that it is not so easy to use these 
bundles as rules for syntactic analysis, 
but to describe the dominance behaviour of 
wordforms they seem to be quite 
appropriate. I won't deal here with free 
modifications (real adjuncts and other 
peripheral elements), although they 
belong, according to the theory, also to 
the bundles. To handle them a special 
mechanism is included in the analysis 
procedure. 
THE PHENOMENON OF OBLIGATORY COMPLEMENTS 
In the valency theory obligatory 
complements are normally regarded as 
special parts of the concept of the verb. 
On this level the notion "obligatory" has 
often been investigated. It is connected 
with the classification "complement- 
adjunct", but there are also optional 
complements and obligatory adjuncts. 
For automatic processing this 
classification is not sufficient: 
H. Somers (1986) showed that a more 
flexible classification lead to better 
results, especially with respect to 
machine translation. Somers referred also 
to the problem that obligatory 
complements can be "hidden" in the text: 
- Ellipses and other phenomena lead to 
omissions which are hard to handle. 
- In modified syntactic constructions 
(passive, nominalisations) complements 
can be omitted regularly. 
- In other constructions the complements 
stand in quite different relations to 
the form derived from a verb (the 
phenomenon of control, attributive 
participles etc.). In these cases the 
complements have to be found by special 
tools. 
Concerning the examples in the first 
paragraph regular omissions are possible 
in (3), (4), (5) and (6) while the 
315 
sentences (2), (6) and (7) belong to the 
third category. They all have to be 
handled in syntactic analysis, but the 
question arises: What is the advantage of 
using the notion obligatory under the 
named circumstances? 
Obligatory in syntactic analysis 
Normally we suppose that sentences to 
be analysed are correct. But, if we 
construct a set of bundles (with 
obligatory edges), we are defining a set 
of sentences which will never be complete. 
If there are no obligatory edges, the 
described set is better covering the set 
of correct sentences. Only very simple 
demands have to be regarded like the 
necessity of the surface subject. In this 
way a parsing system can work quite well. 
In the $aarbrGcken MT-systems a 
dictionary is used where all complements 
are entered in a cumulative way without 
the classification obligatory-optional or 
other relations (Luckhardt, 1985). 
But I think, the possibilities to 
combine complements of verbs (and of 
derived forms) and thus also the notion 
obligatory can be very useful to solve 
ambiguities and to distinguish different 
meanings of a verb. By the way, also in 
SaarbrGcken such mechanisms are used, but 
only in the so-called semantic analysis 
following the syntactic analysis. 
To show the advantages I'll take the 
following verbs as examples: 
a) E@chn@~ 
(I) Er rechnet (die Aufgaben). 
(He calculates (the exercices).) 
(2) Er rechnet ihn zu seinen Freunden. 
(He reckons him among his friends.) 
(3) Er rechnet mit ibm. 
(He takes him into account.) 
In the first case the direct object is 
optional, but the prepositional objects in 
both other cases as well as the direct 
object in the second case are obligatory. 
If not, the first sentence would have all 
three meanings! Only the subject is not 
important for the distinction of the 
meanings, and it is not as obligatory as 
the other complements, because it can be 
omitted by passive transformation. 
b) b_.~e s__~t eh en 
(I) Es besteht Hoffnung. 
(There is hope.) 
(2) Er besteht die PrGfung. 
(He passes the examination.) 
(3) Die Fabrik besteht seit 3 Jahren. 
(The factory has existed for ...) 
(4) Er besteht auf seiner Meinung. 
(He insists on his opinion.) 
(5) Die Wand besteht aus Steinen. 
(The wall consists of stones. 
(6) Das Wesen der Sache besteht darin,.. 
(The nature ... consists in ...) 
Here in (I) and (3) the subject is 
obligatory, but in (2) only the direct 
object. In the other cases the 
prepositional objects are obligatory, thus 
the distinction of the different meanings 
is possible without ambiguities. 
c) erw~rten 
(i) Er erwartet G~ste. 
(He is waiting for guests.) 
(2) Die Kinder erwarten (von den Eltern) 
ein Geschenk. 
(The children expect a gift (from 
their parents).) 
Because of the possibility to form a 
passive sentence from (I), the subject is 
not obligatory in this case. But in (2) it 
is obligatory. Unfortunately the 
distinctive complement with yon is not 
obligatory, thus the distinction of these 
two meanings requires also to take into 
consideration the selective properties of 
the direct object. 
The conclusion of this paragraph can be 
that the classification in obligatory and 
optional complements is only important in 
a final stage of syntactic analysis to 
support the distinction of different 
meanings of wordforms (especially verbal 
forms or forms derived from verbs). But 
this distinction is very useful mainly 
with respect to machine translation, as we 
can see translating the different meanings 
of the examples. 
PRACTICAL CONCLUSIONS 
As we have seen in the first paragraph 
the bundles (i.e. the logical formulas) 
have their place in the lexicon as 
description of the dominance behaviour of 
the wcrdforms. There is no problem, if a 
wordform lexicon (with full forms) is 
used. But in an extensive syntactic 
analysis system a morphological analysis 
has to be included. 
Obligatory in the lexicon 
For a morphological analysis (not only 
an inflexion analysis) we need a lexicon 
of bases and a lexicon of affixes. In the 
lexicon of bases there must be a general 
description of the grammatical properties 
and with the affixes rules have to be 
stated for calculating the properties of 
the derived wordforms. 
What does this mean for the description 
of the dominance behaviour? To calculate 
with the logical formulas seems to be not 
very convenient. 
Therefore the dominance component is 
divided into two parts: The first one is a 
316 
cumulative list of the subordination 
relations and the second one contains the 
bundles. 
For the first part a splitting of the 
subordination relations is advantageous. 
The subordination relations are very 
complex things consisting of different 
kinds of information: 
- usual ideas about syntactic parts of 
sentences like subject, attribute .... 
- paths of action for selective 
connections, 
- paths of action for paradigmatic 
connections, 
- wordclass conditions etc. 
The first two express the well-known 
syntactic functions (SF's), the others 
their a~pearances - so-called morpho- 
syntactic relations (MSR's) - which are 
only necessary to recognize the syntactic 
functions. If a syntactic function is 
recognized, the used morpho-syntactic 
relation can be forgotten. 
Thus this part of the dominance 
component is a list of syntactic functions 
which have pointers to the MSR's 
expressing this syntactic function in case 
of the concerned wordform (SF-MSR-Iist). 
The rules for the derivations concern only 
this list, i.e. only the MSR's under the 
SF's can be changed. For instance: 
rechnen 
SF's MSR's 
SUBJ N-I noun in nominative case 
DOBJ N-4 noun in accussative case 
ZU P-ZU preposition zu 
MIT P-MIT preposition mit 
or S-DASS d_a~_-clause 
or I-ZU infinitive with zu 
(S-DASS and I-ZU only with 
correlate) 
After the passive transformation we 
have the following list: 
SUBJ 
DOBJ 
ZU 
MIT 
P-PRACT prepositional actor 
N-I noun in nominative case 
see above 
see above 
A nominalisation (die ~eQh~g) leads 
to the following: 
SUBJ N-2 noun in genitive case 
or P-PRACT prepositional actor 
DOBJ N-2 noun in genitive case 
or P-VON preposition yon 
ZU see above 
MIT see above 
Thus the bundles are not concerned by 
the rules connected with the derivations. 
But the problem remains how to handle the 
property "obligatory" here. We have two 
possibilities: 
- Only those complements which are 
obligatory in all derived forms are 
marked by the sign OB. In this case, 
the subject is not obligatory for many 
verbs, especially for all transitive 
verbs. Choosing this possibility, the 
"surface obligateness" (e.g. of a 
surface subject) has to be generated 
during the process (depending on 
derivation). 
- All semantically obligatory complements 
are marked by OB. Then changes have to 
be performed during the analysis 
process, too. 
We intend to follow the first way. At 
this point the question arises how to deal 
with the omissions of the third category, 
where the complements are not really 
omitted, but have to be looked for at 
other places within the sentence. That 
means that these complements are not 
connected with the verbal node by a direct 
edge (downward), but - in our theory - 
they are connected by a path of action for 
the corresponding selective connection. In 
this way it is possible to let these 
complements be obligatory and to remark in 
the SF-MSR-Iist that instead of a MSR a 
path af action leads to the concerned 
complement. 
Thus the SF-MSR-Iist for the infinitive 
EeRhnen will have the following form: 
SUBJ via SUBJ-path of action 
DOBJ N-4 
ZU see above 
MIT see above 
As result of the discussion we have the 
following formulas for the different 
meanings of rechnen: 
(I) (SUBJ v DOBJ) 
(2) (SUBJ v (ZU A DOBJ) oB) 
(3) (SUBJ v MITeS) 
Obligatory in the analysis process 
Finally I'll give a short survey of our 
syntactic analysis system to show that the 
bundles and with them also the notion 
obligatory - are used only in the very 
final stage. 
The first step of the procedure is a 
sequential preanalysis (performed by an 
ATN) which has the task to find the 
segments of the sentence and the verbal 
groups of each clause. 
The second step is a local analysis 
where only two nodes and the relations 
between them are regarded. Here the SF- 
MSR-lists are used to recognize the 
possible syntactic functions. 
But in the third step wrong readings 
from the first two steps are filtered out 
using the bundles, i.e. the logical 
317 
formulas, together with the selective 
conditions (transported by the paths of 
action). A side effect of this so-called 
global bundle analysis is the selection of 
the actual verbal meaning. Only here the 
notion "obligatory" is used. 
To conclude this paper I'll emphasize 
once more the problems which have to be 
taken into consideration, if the notion 
"obligatory" is used for syntactic 
analysis: 
- The advantage of using such a concept 
is the possibility to solve ambiguities 
and to s.elect actual meanings of word- 
forms (especially verbal forms). 
This is the reason why it shall be used 
only in a final stage of analysis. 
- The different possibilities to omit 
obligatory complements have to be 
treated in an adequate way. Here 
special procedures during morphological 
analysis and the mechanism of selective 
connections (paths of action) can help 
to handle the regular cases. For other 
omissions (in ellipses etc.) default 
solutions are proposed. 
REFERENCES 
Engel, U.; Schumacher, H. 1978 Kleines 
Valenzlexikon deutscher Verben. TBL 
Verlag Gunter Narr, THbingen. 
Helbig, G.; Schenkel, W. 1983 W~rterbuch 
zur Valenz und Distribution deutscher 
Verben. Verlag Enzyklop~die, Leipzig. 
Kunze, J~rgen. 1975 Abh~ingigkeitsgramma- 
tik. Studia Grsmmatica XIl, Akademie- 
Verlag, Berlin. 
Luckhardt, Heinz-Dirk. 1985 Valenz und 
Tiefenkasus in der maschinellen Ober- 
setzung. CL-Report No. 4, Sonderfor- 
schungsbereich I00, Universit~t des 
Saarlandes, Saarbr~cken. 
Reimann, Dorothee. 1982 B~sehel als syn- 
taktische Regeln. In: Kunze, J0rgen, 
Ed., Automatisehe Analyse des Deut- 
schen. Akademie-Verlag, Berlin. 
Somers, Harold L. 1988 The Need for MT- 
oriented Versions of Case and Valency 
in MT. In: Proceedings COLING'86, Bonn. 
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