Creating inflecting electronic thesauri
Antti Arppe, Mari Voipio, Malene Würtz
Lingsoft, Inc.
Abstract
The contents of Swedish, Danish and Norwegian Bokmål electronic dictionaries were integrated with computerized morphological models for each respective language in order to create inflecting thesauri. Studying and analyzing these outcomes provided practical insight into the workings of lexical generality and synonymity. As a result, it would seem that lexical generality does not apply as universally as one would expect: in addition to categorical, morphology-based restrictions and exceptions there are also pragmatic, common-sense restrictions to it. Furthermore, it would seem that synonymity does not categorically extend and survive to all the respective inflected forms of the original synonyms, i.e. that inflectional features of words should also be taken into account when describing synonymity.
1.	Introduction
The work described in this article was carried out by the authors at Lingsoft, Inc., a language engineering company based in Helsinki, Finland. Lingsoft specializes in providing high-quality natural language processing tools for word processing, text retrieval and information management systems.
One group of such tools is inflecting electronic thesauri, which Lingsoft has developed for the major Nordic languages (in order of the time of completion: Finnish, Swedish, Danish and Norwegian Bokmål). In this presentation we will start with a general overview of what an inflecting thesaurus is and how they were developed by us. Then we will concentrate on linguistic problems we encountered with the inflecting electronic thesauri for Danish, Norwegian Bokmål and Swedish.
2.	What we did
Electronic thesauri are certainly not our invention: any dictionary profits from the search capabilities that a computerized form can provide. Several text processing programs have long had built-in thesauri, but until now they've only worked with the base forms, i.e. they have been facsimiles of the original paper versions of the dictionaries. Lingsoft's novelty is to give the possible synonyms of a word in the same inflection as the input word, which is the definition of an inflecting thesaurus in a nutshell. This feature makes these tools easier to use while writing or editing any normal running text.
Schematically, an inflecting thesaurus works in the following way:
I.	inflected word or phrase & morphological analysis
→  base form(s) of the word
→ inflectional information of the word
II.	base form(s) & data structure containing synonym relationships
→ list of synonyms
III.	lists of synonyms & inflectional information & morphological generation
→ list of synonyms in inflected form

A concrete example of this are the following synonyms given for klassificerades, either a past passive or a genitive form of the past participle of klassificera ‘to classify’:
klassificerades
	ordnades (verb)
delades upp i klasser, indelades, klassades, grupperades,
rangordnades, rankades, inrangerades, graderades,
kategoriserades, systematiserades
	betecknades (verb)
bedömdes
	betecknades (adjectival participle)
bedömdas
The basic ingredients of an inflecting thesaurus is a computational morphological model of a language – in our case the two-level models or TWOLs ‘Koskenniemi 1983’ – and electronic versions of synonym dictionaries – typically licensed from well-known book publishers. The Danish two-level lexicon had been licensed earlier from Thomas Bilgram and developed further by Lingsoft ‘Bilgram 1994’, while the Norwegian Bokmål and Swedish lexicons were developed entirely at Lingsoft ‘Moshagen 1998’ and ‘Karlsson 1992’. The synonym lists were licensed from Politikens Forlag for Danish (Politikens Synonymordbog ‘PSO’), Cappelens Forlag for Norwegian Bokmål (Cappelens Media Synonymordbok) and Walters Lexikon for Swedish (published as Bonniers Synonymordbok).
Developing the inflecting electronic thesauri was a process that lasted several months, partly because of the unexpected linguistic problems that were to be solved for a satisfactory result. Lingsoft's inflecting thesauri for Danish and Norwegian Bokmål were finished in May 1997 and they are included in Danish respectively Norwegian versions of the first upgrade version of Danish respectively Norwegian Office 97, called Office 97A. Lingsoft’s inflecting thesaurus for Swedish was finished already in 1996, and was included in the Swedish version of Office 97.
3.	What we found out
3.1	Concept of lexical generality
Theory on generative morphology postulates that it is ‘in general possible to attach to any word the entire set of inflections associated with the word [base form] in question’ ‘Scalise 1984’. In other words, a set of inflectional categories can be generalized and extended to hold for all – both existing and prospective – members of a particular lexical category or word class. Bybee (1985).uses the term lexical generality for this phenomenon.
For example, all Swedish nouns can be inflected in principle in eight different forms (case, definiteness and number). Thus, were we to form an entirely novel Swedish noun such as rucka, its inflectional paradigm would consequently be rucka, ruckan, ruckor, ruckorna, ruckas, ruckans, ruckors, ruckornas, whatever this word may eventually mean.
In contrast, lexical generality does not apply across all derivation. Thus, from the Swedish verb tävla ‘to compete’ we get tävling ‘competition’, but from finna ‘to find’ we do not get *finning but finnande ‘finding’. Even less so can all words be freely compounded with others.
Any rule-based morphological model of a language, such as a two-level model, can be seen as an incarnation of lexical generality, since such a model incorporates a lexicon of word stems, of which each is attached to a particular lexical category, and sets of inflectional rules, i.e. paradigms, deemed to apply for all the members of a particular lexical category.
3.2	Extent of lexical generality
Thus, an inflecting thesaurus can be seen as the application of lexical generality over the entire contents of the synonym dictionary. Since both computerized morphological models AND electronic versions of synonym dictionaries of the respective languages have co-existed only recently, the effects of this combination on a large scale have not been practically observable until now, which we can see with inflecting thesauri.
During the development of the inflecting thesauri, however, we found out that lexical generality does not always hold. There are certain limits to it, both categorical and pragmatic. To create truly functional, i.e. user-friendly, inflecting thesauri we therefore found it necessary to implement certain grammatical restrictions in the productive inflectional mechanism.
Table 1
	Test1	Test 15	
Total amount of suggestions	2823	1893	
Average amount of suggestions per word	36	24	
Words with no suggestions	13 
(16.6 %)	0	
Wrong word forms suggested	76 
(2.7 %)	3 
(0.16 %)	
Odd word forms suggested	126 
(4.5 %)	7 
(0.48 %)	

To illustrate the necessity of these restrictions, we present in Table 1 some figures derived from the Danish test list, containing 78 randomly picked words from PSO, used to detect mistakes and other irregularities. The suggested synonyms generated in the first test contained quite a few wrong word forms and even more odd suggestions, and one-sixth of the words in the test list received no synonyms at all. The wrong word forms were mostly due to errors in DANTWOL’s inflectional system and were easily corrected or eliminated. The odd suggestions, however, were mostly comparative or superlative forms of adjectives and passive forms of multiword phrases. We only chose to eliminate the ones that sounded very strange to the native ear, but the validity of the rest of these suggestions could certainly be discussed as well.
Categorical limits:
Some verbs simply cannot appear in past passive: *vars, *blevs, *gikks or in imperative: *aftn!, *bero!, as these forms are semantically impossible. As for comparison of adjectives, it cannot be applied to so-called compound adjectives such as kœmpestor, skidegod or non-inflecting adjectives such as ekstra.
Pragmatic limits:
Past passive forms of verbs like binde and lade (that is, bandtes and lodes) are morphologically correct, but sound rather strange to the native user; they are beyond a pragmatic limit. Likewise, imperative forms of the verbs figurere and eksistere seem odd. Norwegians also find some morphologically and semantically correct imperative forms like sykl! quite strange. In this category we also placed the genitive forms of adjectives in singular – stors and storts are in principle correct forms, but they are pragmatically dubious, as are the genitives of past participle singular indefinite forms (spists, elskets). Another example of pragmatic limits is the formation of comparative and superlative forms of certain adjectives - some adjectives describing colours can easily be compared (eg. grønnere), while others are doubtful, to say the least (eg. *orangere).
3.3	Inflectional homonymy
During the testing process, we also became aware of the inflectional homonymy, which is rather abundant in the Scandinavian languages. The testing was carried out by running lists of inflected test words gathered randomly from the original thesaurus sources through the initial electronic versions of the thesauri to ensure that these really were fully inflecting. The output of these tests was then carefully scrutinised in order to detect as many irregularities as possible. An example of inflectional homonymy is a word like flyver that can be a verb in the present tense as well as a noun in base form. Likewise, the word anklagedes can be interpreted as either past passive of the verb anklage or as the definite genitive form of the participle anklaget (or definite/indefinite form plural...).
Inflectional homonymy is not necessarily a problem, for example flyver as a noun and flyver as present tense of the verb flyve are both common and well-known forms. But when some of the inflectional homonyms are common forms and others are very uncommon or unlikely, they look confusing to the user of the thesauri. For example, by typing gås the user most likely means the bird goose and not the present passive form of the verb gå ‘to go’. This is one of the reasons why pragmatic limits HAVE to be drawn somewhere.
3.4	Synonymity and inflection
Another phenomenon that was observed in the creation of these inflecting thesauri was how well synonymity extends with inflection. Traditional theory on synonymity focuses on the semantic traits of words and the grade of synonymity in a semantic sense (absolute versus partial synonymity), but does not really address questions regarding inflection. For instance:
‘… two items are synonymous, if they have the same sense.’ ‘Lyons 1968’
‘Synonyms, then, are lexical items whose senses are identical in respect of “central” semantic traits, but differ, if at all, only in what we may provisionally describe as “minor” or “peripheral” traits.’ ‘Cruse 1986’
‘Synonymity occurs when two terms correspond in all three aspects of designatum, connotation and range of application. […] If the correspondence is exact in one or two of the three aspects but not in all, the two words are near synonyms.’ ‘Zgusta 1971’
Existing synonym dictionaries are by and large composed to represent semantic relationships of the exact word forms that they contain. For the most cases, synonymity appeared to extend quite well into the inflected forms of synonym words. For instance, let us look at the [near] synonymous nouns bok ‘book’ and verk ‘printed work’, and the near synonymous adjectives gammal ‘old’ and möglig ‘moldy’ in the following sentences
Denna bok är gammal. ‘This book is old’
 Detta verk är mögligt. ‘This printed work is moldy’
De här böckerna är äldre. ‘These books are older’
 De här verken är mögligare. ‘These printed works are moldier’
In the above cases, synonymity can be seen to extend to the inflected forms of different declinations of nouns, with bok belonging to the utrum gender and verk belong to the neutrum gender.
On the other hand, quite a few cases of verb synonymity are sensitive to inflection. Take for example hitta ‘to find’ and finna ‘to find’, and säga ‘to say’ and betyda ‘to mean’. The following sentences are more or less synonymous:
	Sen hittade jag boken. ‘Then I found the book.’
	 Sen fann jag boken. ‘Then I found the book.’
	Vad vill det säga? ‘What does it mean? [Literally: What does it say?]’
	 Vad vill det betyda? ‘What does it mean?’
In the following sentences, on the other hand, the inflected forms of finna and säga are no longer appropriate synonyms for various reasons, though they are morphologically interpretable, if not altogether acceptable:
	Boken hittades. ‘The book was found.’
	 Boken fanns. ‘The book existed.’
	Säg någonting! ‘Say something!’
	 *Betyd någonting! ‘Mean something!’
In the first case, inflection of finna into the passive form changes the pragmatic meaning from ‘find’ to ‘exist’ or  ‘there is’. In the second case, inflection of betyda into the imperative form sounds odd to the native Swedish ear.
In the Swedish inflecting thesaurus, this degeneration or discontinuity of synonymity as a result of inflection was observed particularly with the passive and imperative forms of verbs, which therefore had to be manually reviewed. Similar phenomena were also observed with the Danish and the Norwegian Bokmål thesauri. All in all, it was our conclusion that synonymity cannot be categorically expected to extend to all the inflected forms of words originally deemed synonyms, which probably is not a big surprise to most lexicographers. If one would want to present reliable synonyms for passive forms of verbs, for instance, one would have to compose an entirely new synonym dictionary for that purpose, using the passive forms of verbs as the starting point.
4.	Conclusion
Creating inflecting thesauri for Danish, Norwegian Bokmål and Swedish by combining computerized morphological models with electronic versions of synonym dictionaries gave ample practical insight into a few linguistic phenomena, namely lexical morphology and lexical semantics (synonymity) at their intersection. The most essential implications that we discovered in this work are the following:
•	lexical generality does not apply as universally as one would expect: in addition to categorical, morphology-based restrictions and exceptions there are also pragmatic, common-sense restrictions to it.
•	synonymity does not categorically extend and survive to all the respective inflected forms of the original synonyms.

Antti Arppe, Mari Voipio, Malene Würtz
Lingsoft, Inc.
Tehtaankatu 27-29 K, FIN-00150 Helsinki, Finland
antti.arppe@lingsoft, mari.voipio@helsinki.fi, malene.wuertz@helsinki.fi

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