Handling synset overgeneration: Sense Merging in BTB - WN (2021)

Abstract: The paper reports on an effort to reconsider the representation of some cases of derivational paradigm patterns in Bulgarian. The new treatment implemented within BulTreeBank-WordNet (BTB-WN), a wordnet for Bulgarian, is the grouping together of related words that have a common main meaning in the same synset while the nuances in sense are to be encoded within the synset as a modification functions over the main meaning. In this way, we can solve the following challenges: (1) to avoid the influence of English Wordnet (EWN) synset distinctions over Bulgarian that was a result from the translation of some of the synsets from Core WordNet; (2) to represent the common meaning of such derivation patterns just once and to improve the management of BTB-WN, and (3) to encode idiosyncratic usages locally to the corresponding synsets instead of introducing new semantic relations.

## introduction since the first release of princeton wordnet (pwn) @xcite -at the beginning of 1990it has become one of the most popular and used language resources in the field of nlp. it models lexical knowledge as a lexico-semantic network, containing groups of word senses with associated set of synonyms, called synsets, which are related via lexical and semantic relations between them. the need for machine-readable dictionaries made the wordnets one of the most valuable resources for lexical and semantic knowledge for tackling nlp tasks such as word sense disambiguation (wsd), relation extraction, named entity recognition and linking, multi-word expression (mwe) handling, machine translation, etc. pwn also became a model for similar semantic lexicons for many other languages. currently, the global wordnet association hosts information for wordnets for more than 50 languages. 1 these subsequent wordnets are usually created by two approaches called the merge model and the expand model @xcite . in the merge model the monolingual wordnet for the given language is created from scratch -building the set of synsets for the language and interlinking them via set of semantic and lexical relations, without any influence from any previously existing wordnet (for english or other language). if necessary this monolingual wordnet is later mapped to the pwn. examples for wordnets constructed according to this approach (among others) are germanet @xcite , polish wordnet @xcite , norwegian wordnet nornet @xcite and danish wordnet dannet @xcite . in the expand model a new wordnet for the given language is created by transferring lexical knowledge from pwn via translation of synsets, their glosses and semantic relations in a manual or semiautomated way. the expand approach rests on the assumption of the universality of the human mental lexicon and the accuracy of the specific language structure of the internal relation network may be arguable when transferred to another language. examples of wordnets created in this way are multiwordnet @xcite , asianwordnet @xcite , indowordnet @xcite , open dutch wordnet @xcite , slownet for slovenian @xcite , finnwordnet @xcite and the french wordnet wolf @xcite . in both cases if the mapping between a wordnet in a given language to pwn has to be supported, it suffers from two major problems: (1) the conceptual difference between languages -having differ-ent ways of concept lexicalization in the different languages; and (2) the granularity of the senses encoded within the wordnets. the usual way to cope with the first problem is to prepare a rich inventory of interlingual semantic relations. the second is more problematic. in this paper we present a linguistic approach to restructure bulgarian bul-treebank wordnet (btb-wn) synsets in order to escape from the high granularity of senses in pwn. the granularity problem of pwn was reported in @xcite . they discover that some word senses are semantically so close or too fine-grained that even humans have difficulties to make a distinction. the paper proposes a solution to this problem by collapsing similar synsets and removing rarely used senses. this encoding of too fine-grained distinctions was considered to be detrimental for tasks such as wsd and even led to attempts to reduce the granularity of the wordnet sense inventory @xcite . in this paper we propose a linguistically motivated way to join some related senses in btb-wn that are representationally separated. as reported in @xcite bulgarian bultreebank wordnet (btb-wn) has been created by a combination of the merge and expand methods in three different stages: (1) by manual translation of english synsets from the core word-net subset of pwn foot_1 into bulgarian. this step ensures comparable coverage between the two wordnets on the most frequent senses; (2) by identification of senses used in the bulgarian treebank bul-treebank (btb). then these identified senses have been put into newly created synsets for the bultree-bank wordnet and at the same time being mapped onto the conceptual structure of pwn. in this way, the btb-wn was extended with real usages of the word meanings in texts. also, the coverage of the core and base concepts for princeton wordnet has been evaluated over a bulgarian syntactic corpus; (3) by sense extension, which includes two tasks: a) detection of the missing senses of processed lemmas in bultreebank and adding them to the btb-wn, and b) a semi-automatic extraction of information from the bulgarian wiktionary and later from the bulgarian wikipedia mapped to synsets from pwn and then manually checked. in all these steps relation between synsets in btb-wn and pwn have been maintained. there are several reasons to do this, but the two main reasons are: the transfer of information from pwn to btb-wn such as lexical and semantic relations and other; and the support of multilingual applications of btb-wn. the same paper shows the use of several types of correspondence used to ensure that the differences between the two languages are preserved during the mapping between btb-wn and pwn. the full correspondence is observed when the bulgarian and english concepts match perfectly. the next type of correspondence is the partial kind which can be divided into two subtypes depending on the direction. the first one is observed when the bulgarian concept is more specific than the english one. in this case the bulgarian term is mapped to a more general english. the second one is observed when the english concept is more specific. in this case the bulgarian concept is mapped as hypernym of the english. currently btb-wn is under extension and revision. we are doing reconsideration of the granularity of bulgarian synsets in order to represent bulgarian conceptualization of lexical knowledge in a better way and also to establish more appropriate mappings to pwn while reducing the impact that pwn has over the bulgarian. here we are reporting on exploiting derivational morphology in order to establish better definition of bulgarian synsets. the paper is structured as follows: in section 2 some related works discussing similar difficulties in the mapping of wordnets are shown; section 3 presents the strategy for sense merging and some typical cases; and section 4 concludes the paper. ## related work in this section we present some previous works (beside the ones sketched above in the introduction) on mappings between different wordnets and pwn. our goal is to stress the main problems related to the maintenance of mapping between wordnets. when comparing two monolingual wordnets the major challenges are connected to language and cultural idiosyncrasies that affect the granularity of the represented synsets. the most prominent example of supporting mappings between languages with different conceptualizations (sometimes based on differences in the grammatical structure) is the mapping between polish wordnet (plwn) and princeton wordnet. in @xcite the authors explore the problems comparing the two wordnets. the first challenge is related to the different approaches towards the construction of plwn and pwn. the polish synset is rigidly defined as a set of lexical units sharing a set of constitutive relations which makes the plwn more fine-grained than pwn, where synset members are more arbitrary. the second challenge lies in the substantial cross-linguistic differences between english and polish which also reflect on the relation structures of the two wordnets. we identified similar issues in our own work on btb-wn with respect to cross-linguistic differences between english and polish. this became obvious when we translated the corewordnet into bulgarian. it was necessary, for example, to relate some bulgarian adjectives to english nouns, or to provide a bulgarian syntactic phrase to an english lexicalized verb in these languages, etc. we differ from the polish approach to creation of a wordnet in the way of handling the granularity. we would like to minimize the number of semantic and lexical relations introduced by the different kinds of synsets. we believe that such relations become idiosyncratic and do not reflect the richness of sense usages. this motivates the work reported here. to deal with these differences new types of lexical unit relations were added to plwn, such as diminutivity or cross-categorial synonymy (noun-adj for example). the effect on relation structures was that they are partially or wholly different leading to the usage of different types of interlingual relations going far beyond interlingual synonymy defined as simple equivalence by @xcite plus the addition of some new ones. another example for merging two wordnets is the case of the danish wordnet -dannet compared to princeton wordnet in @xcite . the authors present the linking between core concepts in the direction english to danish language, where the majority of links are direct links in terms of manually evaluated exact synonym relations. in the opposite direction when danish is the starting point for the linking process it becomes more complex. in these cases the human annotators had to look for a danish hypernym, and search for candidates for mapping among the related pwn hyponyms or search directly for a potential pwn hyponym. regarding the challenges caused by different sense granularities in the two lexical resources, the danish lexicographers responsible for the mapping of the core concepts reached the conclusion that the sense inventory of pwn is more fine-grained than the one of dannet. the directionality problem applies also to btb-wn when mapped to pwn, but it is outside the scope of this paper. similar problems are found also in the mapping of the romanian wordnet (rown) to princeton wordnet @xcite . the paper reports on the difficulties encountered during the alignment of synsets between english and romanian. the authors define the reasons for these difficulties as the inconsistencies and the difference in criteria for senses recording found both in pwn and rown and the inherent differences in the lexicalization of concepts in the two languages. the romanian wordnet uses a mixed merge-expand model where the romanian senses were collected from different sources and linked to pwn synsets and some romanian synsets were modified to match the linguistic concepts of pwn. the two major challenges reported in this work are: (1) the difference in the granularity between the second language (romanian in this case) and the english language, and (2) the conceptual gaps in one or the other language. the paper also puts forward a rarely discussed problem -that mapping a non-english wordnet to pwn is often done by non-native english speakers, who encounter difficulties in correctly perceiving the english senses especially in the fine-grained structure of the pwn. similar observations were made also by us. thus, further development in mapping might require involving native english speakers. the granularity problem appears in the case of the turkish wordnet -kenet @xcite . it is built as a monolingual resource on the base of the latest contemporary dictionary of turkish with its semantic relations suggested by pwn, but manually checked and only the correct relations added to the kenet. before the addition of sense relations the turkish synsets underwent a merge and a split processes. in the merge process "different synsets that should be grouped together were identified and grouped as a single synset". during that process three important rules were established: (1) each synset has to have one single and unique definition, (2) only true synonyms are valid synset members, and (3) a representative lemma for the whole synset must be a first member in each synset. in the split process wrongly grouped senses were identified and separated into new synsets. this allowed the creators of kenet to control the synset members and identify which synsets had missing members or had extra members. we use a similar approach to reconsider the structure of btb-wn and its mapping to pwn. the main difference is that we try to be consistent with respect to whole sets of synsets in bulgarian motivated by linguistic knowledge about related senses as it will be clear in the next section. ## linguistically motivated sense merging currently btb-wn is being developed in two directions: the following subsections will outline several cases in which sense merging is the preferred choice instead of creating several very similar in meaning synsets. as we stressed earlier we use derivational patterns in bulgarian for this purpose. ## conclusion and future work the paper presents an attempt at dealing with sense merging in the process of mapping two wordnets with a focus on the treatment of synsets for diminutives, verb forms with different ways of action and female and male forms of nouns for professions, roles, nationalities and animals. the work reported here is based on the notion that making the coverage of btb-wn based on number of lemmas (synset members) instead on number of synsets will be beneficial when applying wordnet data to the tasks like wsd. as a secondary result of this work several suggestions for edits in the english wordnet were made. in the future we hope to achieve even larger coverage of lemmas by including dialectal, archaic, slang, diminutive and vulgar words as new synsets or new synset members. one task still remaining is the ordering of the synset members relying on the notion that the first member has to be the exemplar. we have reported some works on the mapping mainly of wordnets for languages from indo-european language families to english pwn. it would be interesting to extend the study for mapping between different languages within the family (for example bulgarian to polish or bulgarian to german) and also between wordnets for languages from different language families. we are aware of the even greater problems with the mapping between asian wordnets and the english one.