EUEOTRA 
PRACTICAL EIPERIKI~K WITll A MOLTILINGUAL NACHINE T~LATION SYSTI0m UNDER DI~ELOPMKlqT 
Giovanni B. Varlle S Peter Lau 
COmmiSSiOn of the European Cou~unltles 
J. N onnet Bldg. B4/002 
L-292G Lusembour9 
£BSTUACT 
In this paper we will motivate the 
design decisions for the architecture of 
the Eurotra system and Its grammar 
formalism In terms of the Intrinsic 
constraints of multlllngual machine 
translation and the extrinsic requirements 
Imposed by the specific organizational 
structure of the project. We will give an 
account of the state of Implementation of 
the system to date and assess advantages 
and disadvantages of Its framework In the 
light of the Implementation. 
INTBODUCTION 
This paper tries to glve an up-to- 
date picture of the Eurotra project as of 
the third quarter of 1987, both concerning 
the state of the Implementation of the 
system and a critical evaluation of the 
Eurotra formalism. 
The paper Is structured as follows : 
In Section i we briefly summarize the 
purposes and organization of the project 
and the consequences for the design of the 
Eurotra framework. 
In Section 2 we outline and motivate 
the distinctive aspects of the framework 
in view of the extrinsic requirements 
outlined in Section i and the Intrinsic 
requirements of multlllngual machine 
translation (NMT). 
In Section 3 we give the linguistic 
assumptions underlying the first small- 
scale Implementation, which will be 
described in some detail In Section 5. 
Section 4 contains an evaluation of 
the framework, both for Its positive and 
Its negative aspects, in the light of the 
Implementation carried out to date. 
In Section 5 we describe the design 
modifications carried out in order to 
overcome some of the problems spotted 
during the Implementation and report on 
the recodlng effort which was necessary 
to-date to adapt the original grammars to 
the modified framework. 
I. GOALS AND ORGANIZATION OF Tim ETJROTRA 
PROJECT 
Eurotra Is the European Community 
machine translation (MT) R&D programme. It 
alms at producing by 1990 a relatively 
small multlllngual prototype MT system 
able to translate between all nine 
official Community languages a limited 
number of text types in a limited domain, 
with a lexicon of ca. 20000 entries per 
language. 
The text types Eurotra alms to 
translate may be defined as typical 
administrative texts /memos, minutes of 
meetings, communications etc.) In the 
domain of IT. The system will be truly 
multIllngual In the sense that neither 
analysis of source languages nor synthesis 
of target languages will depend on 
information which is not exclusively 
pertaining to the source, respectively 
target languages In question. As a 
consequence, the addition of new languages 
will not Imply any modifications to the 
already existing analysis and synthesis 
modules, but only the creation of the new 
transfer modules needed, 
Eurotra Is a large programme, 
Involving over 160 people located In 20 
centers spread all over the European 
Community. While this decentralized 
organizational scheme has obvious 
drawbacks, It Is the only possible one 
which allows us to get all the linguistic 
and especially language competence 
necessary for a successful completion of 
the programme without a brain drain from 
national centers which would go against 
the second 9oal of the programme : 
spreading natural language processing 
(NLP) expertise across Europe. 
In this respect, Eurotra has already 
been successful in that it has contributed 
to a larger awareness amongst European 
computational linguists of the need and 
possibility of a continent-wide 
collaboration in the field while promoting 
the awareness in governmental bodies In 
Europe of the growing economical and 
cultural Importance of basic and applied 
research in NLP. 
160 
Furthermore, Eurotra contributes 
directly In broadening the computational 
linguistic competence In Europe, having to 
train a number of people without previous 
competence In NLP in order to get the 
manpower that the project needs for a 
successful completion. 
Eurotra's decentralized organization 
had a strong influence on the design of 
the system, be It the linguistic 
metalanguage or the underlying software, 
because of the necessity of integrating in 
an easy way modules produced by people 
wlth wldely differing linguistic 
backgrounds In geographically distant 
places. We will elaborate on these 
consequences In the next section, devoted 
to the motivation of the design decisions 
of the system. 
More details on the Eurotra 
organizational structure can be found in 
\[5\],\[6\] and \[9\]. 
2. BASIC DESIGN DECISIONS UNDEELYIN6 
TI~ KUROTRA F~RX 
The research orientation of Eurotra 
is motivated by the Intrinsic experimental 
nature of MMT. On the other hand, we are 
committed to produce a working prototype 
MMT system by the end of the programme, 
whlch should serve as a basis for the 
development by industry of the next 
generation of MT systems. These facts, 
together wlth the previously mentioned 
size and decentrallzatlon of Eurotra, have 
determined the design of the system, be It 
the linguistic recta-language or the 
underlying software. 
The Eurotra framework has been 
described in detail In previous papers 
{\[t\],\[2\],\[i2\]). Here we will only briefly 
summarize its distinctive characteristics. 
The central problem in multlllngual, 
transfer-based MT derives from the fact 
that the number of transfer components of 
such a system grows geometrically with the 
number of languages covered (72 transfer 
components for 9 languages}. In order to 
be feasible at all, such a system requires 
that the transfer components be Kept 
small, in principle llmlted to the lexical 
component. Our problem can be formulated 
as follows : 
what ls the nature of the 
representatlon of a text guaranteeing 
translational adequacF ("good quality 
translation") while allowing simple 
transfer ? 
The answer to this question Is not 
Known, and our central effort In defining 
the Eurotra framework was to create an 
experimental environment in which research 
could be carried out in order to find 
possible answers. 
This led us to the design of a system 
based on multiple, successive 
representations and mappings between them, 
leaving the exact definition and the 
number and nature of such levels to be 
determined experimentally. 
Generators 
The set of legal representations for 
each level is defined lntenslonally by a 
generator (in the classical sense} 
consisting of a set of augmented context 
free {CF) structure bulldlng rules, or B- 
rules, complemented with a set of feature 
percolation principles, feature rules or 
F-rules, and a set of filters, called 
Killer rules or K-rules, to control 
overgeneratlon. 
These rule systems have a declarative 
operational semantics based on 
unification, not unlike other current 
grammar formalisms (C3\], C4\], CT\], \[iO\], 
\[II\]|. 
But while some of these formalisms 
explicitly blur the distinction between 
structural Information and feature 
Information (\[3\], \[7\]), we declded to Keep 
them strlctly separated, like the 
formalisms described In \[4\] and \[iO\], 
whlle allowing only one level of recursion 
In features (set valued features) for 
reasons of simplicity, formally not unlike 
the SUBCAT feature of \[10\]. 
This grammar formalism provides us 
with an easily understandable, powerful 
and uniform language and, as a 
consequence, the training of new members 
of the Eurotra groups poses no problems, 
an essential asset In a proJect with some 
160 participants. Furthermore changes of 
the virtual machine become acceptable as 
long as the rule type remains essentially 
the same, because CF based grammars are 
normally easy to revise. 
The current formalism includes 
devices such as the Kleene star, 
optlonallty marker, alternation and 
negation. 
Figure 1 shows examples of the type 
of rules currently implemented, with a 
slightly simplified syntax. 
Translators 
A representation at a given level Is 
mapped onto a representation at the 
adjacent level by a translation rule {T- 
161 
:b-rule: 
pp = { cat = pp | 
\[ { sf -- gov cat = prep | 
{ sf = compl cat :np | \] 
:f-rule: 
prep-feat = { pform : X cat = pp ) 
\[ ( lu : X cat -- prep J 
{ cat -- np | \] 
:K-rule: 
s_mod = { cat = s | 
\[ n ( | 
{ sf -- rood pform = of J m { | \] 
(this last rule expresses the fact that 
sentences cannot be modified by of-PP) 
Figure 1 : Examples of rules 
rule). The translation rules must satisfy 
two criteria : 
they must be simple : It must 
be easier to relate two adjacent levels of 
representation than to relate text to the 
semantic representation Input to transfer; 
furthermore, a source level representation 
Is mapped directly onto a (set of) target 
representations without Intermediate steps 
or representations : we call this the 
one-sl~ot translatJon pr~ncJple; 
they must be composJtJonaI : 
the translation of an object must be a 
(simple) function of the translation of 
Its parts; this In order to, on one hand, 
make the writing of translators modular, 
and, on the other hand, make the relation 
between two representational objects 
established by a set of translation rules 
understandable for the linguist. 
Figure 2 shows two examples of 
translation rules as currently 
Implemented. The first one Is extracted 
from the English to German transfer 
module, while the second one Is part of 
the Italian analysis component. 
tit : { sf=gov cat=v argl feat=collective 
arg2_feat=abstr nonhum lu=verabschleden | 
:> adopt 
tI5 = { cat=s coord=yes | 
\[ St( cat=s coord=no | 
CONG{ cat=conJ } 
$2| cat:s coord:no J \] 
=> coord( CONG St $2 ) 
Flgure 2 : Examples of translator rules 
3. EUROTR£'S LINGUISTICS 
The generators for the first small 
Implementation have been defined In such a 
way that they mirror traditional 
linguistic Ideas about analytical levels : 
morphology, surface syntax (Immediate 
constituents and syntactic functions), 
deep syntax and semantics. However, In 
order to offer a full treatment of all 
texts In our text type and domain without 
pre-edltXng, we also Included generators 
which cater for character normalIsatlon 
(in order for the dictionary to be 
Independent of typography) and text 
structure (e.g. lay-out, text format, 
figures, footnotes). 
At present the linguistic 
specifications define 6 levels In the 
Eurotra analysis and synthesis modules : 
ETS 
ENT 
ENS 
ECS 
ERS 
IS 
Eurotra Text Structure) 
Eurotra Normallsed Text} 
Eurotra Morphological Structure} 
Eurotra Constituent Structure) 
Eurotra Relational Structure) 
Interface Structure) 
Figures 3 and 4 show an ECS, 
respectively IS, representation for the 
sentence : "The decision adopted by the 
Council on 25 April i983 was Implemented 
by the Member States In the course of 
i983". 
The ECS representation of Figure 3 Is 
reduced structurally through two 
steps.First It ls translated Into the 
relational representation ERS which 
Identifies syntactic functions, elevates 
determiners, auxiliaries and valency bound 
prepositions and rearranges the 
constituents Into a canonical order. 
Then the relational representation Is 
translated Into the Interface structure IS 
whereby passive Is undone, empty elements 
are Inserted for oblxgtory arguments which 
are absent In the surface form and 
semantic Information Is added to the 
feature bundles. 
Note that, although the IS 
representation Is fairly simple from a 
structural point of view It still allows 
for modifier attachment at different 
levels of embedding, and thus, the two 
TIME constituents are attached to their 
proper governors without the use of 
complicated featurlsed references. 
All generators being described In the 
same formal language, the representations 
of text structure, normalXsed text and 
morphology are built by augmented CF 
rules, Just like the syntactic 
representations. 
162 
Det 
NP 
NP 
N V 
SBAR 
PP 
Prep NP 
N 
PP 
Prep NP 
N 
S 
Aux 
V 
VP 
PP 
Prep NP Prep 
NP 
N 
PP 
NP 
NP PP 
Det N Prep 
,, 
NP 
NP 
Date 
,, 
the decision adopted by council on 25 were implemented by Member In the course of /983 
April States 
i983 
Figure 3 : Example of an ECS representatlon i 
GOV AR6 i ARG 2 
60V 
GOV ARG I 
TIME 
MOD GOV NOD 
s : : 
Prep GOV 
ARG2 
implement member state declslon(i} adopt council ( 
TIME 
Prep 
,, 
GOV 
on 1982.04. 25 1983 In course 
Flgure 4 : Example of an IS representation / 
The modular approach to linguistics, 
whereby text format, morphology, syntax 
and semantics are handled by separate 
generators, enhances the repairability and 
extenslblllty of the system. This means 
iThese trees are given In the 
form whlch ls output by our parametrlzable 
prettyprlnter with the parameter set to 
the "cat" feature In Figure 3 and to the 
"st" \[for semantic relation} feature In 
Figure 4. Representations Including all 
feature information are much more complex 
and Impractical to read. 
for Instance, that changes of the ETS, ENT 
and EMS grammars may be made freely as 
long as they produce the output needed by 
ECS. 
The sequence of representations 
related by T-rules spans the distance 
between the actual text and the Interface 
structure (IS} which Is the beginning and 
end point of transfer. 
In order to simplify transfer, IS 
abstracts away from surface phenomena like 
Inflection, derivation, compounding, 
constituent structure etc. It contains 
semantically labelled arguments and 
modifiers related to a predicate, and the 
163 
transfer dictionaries, Ideally, Just 
relate lexlcal unlts of one IS to lexlcal 
units of another. 
In some cases, though, this does not 
worE, because of, e.g., lexlcal holes like 
the German word "Schlmmel" (meaning "white 
horse"} which has no lexlcal correspondent 
in English, or different functions mapping 
onto one another as the English predicate 
"liKe" which maps onto a 6erman adverbial 
modifier "gern". In these cases explicit 
non-lexlcal T-rules must be written for 
transfer. 
The purpose of the IS experimentation 
In Eurotra Is precisely to minimize the 
number of expllclt transfer T-rules and 
the entire modular design as it has been 
proposed In the linguistic specifications 
Is primarily geared towards an 
experimentation process aimed at making It 
possible to reach an optimal IS through 
multiple cycles of prototyplng. 
4. EVALUATION OF Tim ¥~RE 
The architecture of our framework has 
given us full satisfaction with respect to 
Its modulaz'lty, 3Jmpilclty and the 
ease wlth which we could modify certaJn 
de31gn characteristic3 of the system, as 
described In Sectlon 5. 
Its modularity has made It 
possible to experiment with the interface 
structure quite Independently from the 
rest of the system. For Instance, research 
and experimentation about an adequate 
treatment of time and modalIty could be 
done In parallel with, and independently 
of, grammar Implementation work concerning 
other levels of representation. 
The simplicity of the generator's 
formalism has had positive and negative 
aspects. Amongst the former we can mention 
the fact that It was eaJ¥ to leaFn and 
teach, easy to r.odJf¥ and a 
reasonably good communication tool for 
the scientists Involved In the definition 
and Implementation of the system. 
On the negative side we must mention 
the fact that It was not expressive enough 
IThls Is true even considering 
the fact that mechanisms for treating 
unbounded phenomena, for expressing rules 
In an ID/LP format and for built-In 
feature Inheritance were given second 
priority, and, therefore, were not 
implemented In the first version of the 
frameworK, nor In the first revision of 
the framework reported here. 
to describe In a natural way all the 
phenomena occurring In the languages we 
are treating / . Furthermore, for a 
system which requires the T-rules to be 
compositional, the operational semantics 
of generators turned out to be too poor, 
causing a proliferation of complex T- 
rules. 
For the Implementation worE, thls has 
caused some problems. For Instance, we 
couldn't Implement ETS, ENT and EMS In the 
first cycle because the prototype only 
allowed structure manipulation to happen 
at one level. This meant that we could not 
build text structure, morphological 
structure and syntactic structure 
Independently, they all had to be built by 
one generator, which then became very blg 
and difficult to manage. 
Another problem was that the 
analytical strategy of elevating 
functional elements like articles and 
prepositions, which Is motivated by the 
needs of MMT rather than by linguistic 
considerations, could not easily be 
reversed, because building new nodes In 
synthesis to represent these elements Is 
addition of structural Information. 
In consequence, we had to change the 
specifications of the virtual machlne In 
such a way that each generator had the 
power of completing a representational 
object according to Its own definition of 
well formed representation, thus 
alleviating the task of the translators. 
The modifiability of our original 
framework was Invaluable In this redesign 
since It allowed us to keep the core 
concepts basically unchanged while 
extending the functlonallty and 
expressiveness of the formalism. 
5. DHSIGN MODIFICATIONS TO THE PRAME~0RI 
As reported above, the first 
Implementation showed that two factors 
were responsible for the heaviness of the 
translators (not to be confused with 
transfer) : the relatively simple 
operational semantics of the generators 
comblned with the requirement that the 
translators be compositional. 
Rather than giving up 
compositionality, which guarantees a well 
defined relation between representations 
belonging to adjacent levels, we increased 
the power of the generators by modifying 
their operational semantics to Include a 
controlled form of addJtion of 
structural Information, rather than Just 
addition of feature Information. 
Translators were impoverished to the 
164 
point where they can now be defined by 
default almost everywhere by specifying 
correspondences between feature theories 
pertaining to adjacent generators. 
Special, exceptional cases of T-rules have 
still to be specified explicitly. ." 
In the modified framework, the 
representation output by a translator is 
completed by the target generator. For 
Instance, ECS will eventually expect as 
input from the preceding level, which Is 
EMS, a tree wlth the top node T(ext| 
branching Into CihapterL Se(cttons| and 
Piaragraphs|. The P nodes should then 
branch directly Into wordform nodes which 
dominate tree-structure representations of 
the morphological structure of each 
wordform. The ECS generator will then 
complete this tree by Inserting S nodes 
and nontermlnal categorlal nodes of the 
constituent structure representation of 
each sentence, resulting In a parse tree 
of the sentence. 
From our original framework we retain 
the architecture, i.e. breaking up the 
monollngual components of the system Into 
a sequence of generators whlch are related 
by translators. In principle the same 
generators are used for analysls and 
synthesis, but we don't know yet whether 
the non-default translators can also be 
used in both directions. 
The first implemented prototype based 
on the revised framework has been used for 
experiments wlth rewriting the grammars of 
the first Implementatlonal cycle, and 
these experiments have confirmed the 
assumption that recodlng of grammars 
does not pose special problems. 
Here It must be mentioned that an 
alternative modlfled framework has been 
proposed to overcome the Inadequacies 
discovered during the first cycle of 
Implementation, which departs more 
radically from the original one. The 
testing that this alternative revised 
framework has undergone has been more 
limited for various reasons. The merits 
and demerits of the two proposed revlslons 
will he assessed during the first quarter 
of i986 In a controlled experiment. 
7. CUHBKNTLY IMPLKIKNTBD STSTKN 
The majority of the implementation 
work to date has been carrled out within 
the original Eurotra framework leading to 
a system coverlng to varying degrees the 
orlglnal seven languages foreseen, that 
Is Danish, Dutch, English, French, 
German, Greek and Italian. Work done on 
Portuguese and Spanish Is scheduled In a 
different way, glven that Portugal and 
Spaln became members of the Community 
only in 1986. Analysis modules exist for 
all languages, generation for flve 
languages. Transfer components have been 
written for the following ten language 
pales : 
Danish - English 
English - Danish 
English - Greek 
German - Danish 
German - Greek 
Danish - German 
English - German 
French - Greek 
German - English 
Greek - English 
An average grammar has ca. 400 rules 
and 500 iexlcal entries |accounting for 
ca. 3000 full-word forms in moderately 
Inflected languagesL 
Before the end of 1987 It Is expected 
that all already implemented components 
will be recoded In the new formalism, that 
ten more language pairs will be added 
while the lexlcal and linguistic coverage 
will he Increased. 
Only the levels ECS, EBS and IS have 
been Implemented to date. Thls means that 
In the first Implementation each 
monollngual component works on the basis 
of a full-form dictionary, since the 
generator for morphology was given second 
priority, and accepts only single 
sentences as Input. 
The linguistic coverage lncludes 
maln clauses and relatlve clauses; all 
types of noun phrases; simple 
coordination In noun phrases; all 
verbal tenses (excluding modal 
constructions); possessive, relative, 
reflexive and Indefinite pronouns; all 
prepositional phrases; ad v e r b s; 
numerals; particles. 
Research and experimentation are 
still ongoing to Include ellipsis; 
modality; negation; scope; quantification; 
time-tense relation and pronoun 
resolution, but we do not expect to be 
able to do pronoun resolution on the basis 
of real world Knowledge in the near 
future. 
The implementation of the virtual 
machine is done mainly In C-Prolog. A new 
version of the software Including a 
relational data base for the coding and 
maintenance of the lexicon (to be extended 
to grammars) has Just been released. 
More details on the philosophy of the 
software construction can be found In 
\[53. 
A fragment of a sample grammar for 
English Is shown In Appendix A. Some 
examples of T-rules are given In Appendix 
B. 
165 
Future ¥org 
By mid 1988 we plan to have a small 
scale, corpus based, prototype with a 
coverage of 2500 lexlcal entries per 
language for all the seven original 
languages together wlth all the 42 
transfer components. 
In the shorter term, several 
additions to the new framework are 
foreseen, the most Important of which 
concern the possibility to express grammar 
rules and T-rules In an ID/LP type format 
\[4\] and a mechanism for treating unbounded 
phenomena. 
Current research topics In 
linguistics have been mentioned above. In 
relatlon to the framework, we are 
currently Investigating about a feature 
Inheritance mechanism and some restricted 
form of complex features other than the 
set valued features mentioned In Section 
2. 
CONCLUSIONS 
In this paper we have reported on the 
organization and current state of 
Implementation of the Eurotra project. We 
have briefly summarized and motivated the 
design decisions underlying Its formal 
framework, reporting on the Inadequacies 
discovered during the first implementation 
work. 
We have argued that the basic design 
was adequate In that It supported the 
experimentation on the interface 
structure. Furthermore, where the design 
had to be modified It could be, and the 
necessary receding of grammars did not 
cause major problems. 
Finally, we have mentioned further 
modifications forseen in the future and, 
based on our past experience, we are 
confident that we will be able to carry 
them out in a way which will not cause 
significant disruption of the grammar and 
lexicon coding work. 
ACF~NOWLKI~3 Ml~gl~fS 
The work reported In this paper Is 
the result of a collaborative effort of 
many people over a number of years. We 
cannot claim exclusive authorship of any 
of the Ideas exposed In the paper. Any 
omission, Imprecision or error are of 
course our sole responsablllty. 

APPENDIX A Sample Grammar Fragment 
Zthls Is a sample fragment of a ECS grammar for English as Implemented 
Zcomment lines start with a "Z" 
:level:ecsEn 
7.this declaration states that the grammar Is a fragment of an English ECS grammar 
:b: 
2this declaration starts the b-rule section of the ECS grammar 
si = \[cat=s type;maln tense=T volce:V mode:M} 
\[ {cat=np} \[cat=vp tense=T voice:V mode:M} ^\[cat=np\] ^{cat=pp\] \] 
vpl = \[cat=vp tense:T volce=Y mode=N} 
\[ \[¢at=v tense:T volce=Y mode:Ni ^|cat=np\] ^|cat:ppl ) 
vp2 = \[cat=vp tense=T voice=pass mode=M} 
\[ |cat=aux v type:flnlte tense=T mode--N lu:be\] 
\[cat=v v_type=ptc\] ^|cat=np\] ^|cat=pp\] \] 
npi : |cat=np n__type=T pers=P num=N case=C def=D\] 
\[ ^|cat:detp de~:D\] .{cat=ap def=D\] |cat=n n_type=T pers=P num=N case=C} \] 
ppt = \[cat=pp prep=L} 
\[ |cat=prep iu=L\] Icat=np\] \] 
apt = \[cat=ap degree=D} 
\[ |cat=adJ degree=D} \] 
detp : \[cat=detp def=D} 
\[ |cat=det clef=D} } 
APPENDIX D Sample T-rules 
ZT-rule between ECS and ERS for elevating a determiner; 
7.capltal letter symbols In front of feature bundles are indices (names} which can be 
Zreferred to In the right hand side of a T-rule for coding economy 
tnpt = NP |cat=np\] 
\[ (cat=detpI A*|cat=ap} N(cat=n\] \] 
=> NP \[ N A \] 
ZT-rules between ERS and ECS for eliminating a VP node 
tsl : S |cat:s type=main} 
\[ NPi\[cat=np\] {cat=vp\] 
\[ V|cat=v\] NP2 ^\[cat=np\] PP ^|cat--pp\] \]\] 
=> S \[ V NPI NP2 PP } 
167 

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