O nto L ex as a possible Bridge between W ord N ets and full lexical Descriptions (2019)

Abstract: In this paper we describe our current work on representing a recently created German lexical semantics resource in OntoLex-Lemon and in conformance with WordNet specifications. Besides presenting the representation effort, we show the utilization of OntoLex-Lemon to bridge from WordNet-like resources to full lexical descriptions and extend the coverage of WordNets to other types of lexical data, such as decomposition results, exemplified for German data, and inflectional phenomena, here outlined for English data.

## introduction we aim at publishing german wordnet conformant data in the linguistic linked open data (llod) cloud. @xcite we selected the ontolex-lemon model (cimiano et al., 2016), a successor and world wide web consortium (w3c) standardization of the lemon model (mccrae et al., 2012b), in order to represent different kinds of lexical semantics data, since it has established itself as the defacto community standard for representing lexical data in the linked data framework. guidelines for mapping global wordnet formats to a lemonbased resource description framework (rdf) representation have been published 2 and already some wordnets have been mapped to lemon, as described for example in (mccrae et al., 2014). a candidate for representing german lexical semantics data in ontolex-lemon is germanet, which is a manually designed wordnet resource for german (hamp et al., 1997). developed more than 20 years ago, it represents a very stable, well-tested, and precise lexical semantics resource. however, its access is restricted by its current license. without such open data access, reuse of germanet in global initiatives, such as open multilingual wordnet (omw) (bond and paik, 2012), is inhibited. one of the objectives of this paper is to represent lexical semantics data openly linked to other omw datasets in the llod. two alternatives compliant with wordnet specifications and available under an open-source license are the lemonuby set of resources (eckle-kohler et al., 2015) and the open-de-wordnet (odenet) effort. @xcite lemonuby is an export of lexical data from the large-scale linked uby (gurevych et al., 2012) foot_3 , which unites collaboratively and expert-developed resources (e.g. framenet and wiktionary) in english and german, to lemon. lemonuby contains the german version of omega-wiki foot_4 , which encodes wordnet compliant descriptions of german words. odenet provides a german resource to the omw initiative. a mapping from lemonuby to ontolex-lemon can be expected to be straightforward due to a high compliance between both models. thus, this publication concentrates on mapping the wordnetcompliant xml code of odenet to ontolex-lemon, while in the long run a cross-linking or, where possible, a merging of lemonuby and odenet in the llod is foreseen. we exemplify the richness of lexical descriptions offered by ontolex-lemon with the case of components of compounds, german in this submission, and inflectional morphological variations, here in the case of sense variations across english nominal plural inflections. in the following sections we first describe odenet, before presenting the main characteristics of ontolex-lemon. in section 4 we present the current state of the mapping from odenet to ontolex-lemon, before finally discussing the potential added-value of having wordnets represented in ontolex-lemon. ## odenet odenet combines two existing resources: the openthesaurus german synonym lexicon 6 and the open multilingual wordnet (omw) @xcite (bond and foster, 2013). in terms of english resources, it includes the princeton wordnet of english (pwn) (fellbaum, 1998). integrating openthesaurus in odenet means making use of a large resource for german that is generated and updated by the crowd. a consequence of this approach is that odenet needs to be curated, as the authors of the resource mention. we downloaded the most recent version 8 and first analyzed its content. odenet is in an xml format and shares its document type definition (dtd) @xcite with other wordnets in the omw initiative. lexical entries provide information on different senses of a lexeme, such as "kernspaltung" or "kernfission" (nuclear fission) in the same synset: <lexicalentry @xmath0, i.e., groups of word senses with the same meaning. hierarchical relations are introduced as synset relations, such as here a hypernymy relation: <synset @xmath1:@xmath2: <lexicalentry @xmath3.0"> <lemma @xmath4. access to the lemma information for hypernyms and hyponyms is also possible, for instance for the odenet-49-n synset for "stuhl" it would be: the ontolex-lemon model was originally developed with the aim to provide a rich linguistic grounding for ontologies, meaning that the natural language expressions used in the description of ontology elements are equipped with an extensive linguistic description (mccrae et al., 2012a; cimiano et al., 2016). this rich linguistic grounding includes the representation of morphological and syntactic properties of lexical entries as well as the syntax-semantics interface, i.e., the meaning of these lexical entries with respect to an ontology or to specialized vocabularies. the main organizing unit for those linguistic descriptions is the lexical entry, which enables the representation of morphological patterns for each word and/or affix. the connection of a lexical entry to an ontological entity is marked mainly by the denotes property or is mediated by the lexical sense or the lexical concept properties, as this is represented in figure 1 , which displays the core module of the model. ontolex-lemon, as well as its predecessor lemon, have also been deployed for the representation of wordnets, as described for example in (mccrae et al., 2014) and guidelines are available for mapping wordnets to an rdf code compliant to ontolex-lemon. @xcite a main difference between lemon and ontolex-lemon is that the latter model includes an explicit way to encode conceptual hierarchies, using the skos standard. @xcite as can be seen in figure 1 , lexical entries (lemmas) can be linked, via the ontolex:evokes property, to such skos concepts, which can represent wordnet synsets. this structure is paralleling the relation between lexical entries and ontological resources, which is implemented either directly by the ontolex:reference property or mediated by the instances of the ontolex:lexicalsense class. @xcite the ontolex:lexicalconcept class seems to be best appropriated to model the "sets of cognitive synonyms (synsets)" 13 that (pwn describes, while the ontolex:lexicalsense class is meant to represent the bridge between lexical entries and ontological entities (which do not necessarily have semantic relations between them). more recently the ontolex-lemon model has been more and more considered also for modeling lexical data as such, in the context of projects and studies related to the development of digital lexicography, like for example in the past cost action "enel" (european network of e-lexicography). @xcite this development towards a more generic representation model for lexicographic purposes is documented among others in (mccrae et al., 2017). ## mapping odenet to ontolex-lemon one main issue that occurred due to partly crowdsourced data in odenet was that additional textual information or special characters were added by the crowd to the headwords. a second issue was the improper use of part-of-speech (pos) tags if word classes were different from noun, verb, or adjective or could not be clearly assigned to ture and content of concept schemes such as thesauri, classification schemes, subject heading lists, taxonomies, folksonomies, and other similar types of controlled vocabulary" ( https://www.w3.org/tr/skos-primer/ ) @xcite quoting from section 3.6 "lexical concept" https: //www.w3.org/2016/05/ontolex/ : "we [...] capture the fact that a certain lexical entry can be used to denote a certain ontological predicate. we capture this by saying that the lexical entry denotes the class or ontology element in question. however, sometimes we would like to express the fact that a certain lexical entry evokes a certain mental concept rather than that it refers to a class with a formal interpretation in some model. thus, in lemon we introduce the class lexical concept that represents a mental abstraction, concept or unit of thought that can be lexicalized by a given collection of senses. a lexical concept is thus a subclass of skos:concept." @xcite quoted from https://wordnet.princeton . edu/. the following listings provide details on the ontolex-lemon encoding of the first odenet entry, which is "kernspaltung" (nuclear fission). in listing 1 we display the full ontolex-lemon entry, which allows us to represent the components of compound words by encoding information as a ontolex:multiwordexpression instance. this class marks any type of entries that can be segmented, thus, including compounds. the term "kernspaltung" is associated with its two components "kern" and "spaltung". each component represents a full lexical entry with all of its semantic relations. reuse of components across ontolex-lemon entries reveals relations between different instances of ontolex:multiwordexpression based on their component entries. this possibility demonstrates one of the added-values of linking synsets to the (complex) representation of lexical entries, as we can state (see below) semantic relations be-tween synsets associated to the components of a compound word and its synsets. listing 2 below displays the form information associated with entry :entry_w1 in listing 1. listing 2: the ontolex:form "kernspaltung" : form_w1 r d f : t y p e o n t o l e x : form ; o n t o l e x : w r i t t e n r e p " k e r n s p a l t u n g "@de ; . listing 3 shows the conversion of the original odenet sense information to an instance of the ontolex:lexicalsense class. listing 3: the lexicalsense associated to the entry for "kernspaltung" a sense can be linked to a synset via the property ontolex:islexicalizedsenseof, which relates a lexical sense to that lexical concept it lexicalizes, here a synset. the entry can be linked to the synset via the property ontolex:evokes, as displayed in listing 1, which is defined as relating a lexical entry to one of the abstract lexical concepts that a speaker of the language would associate with the words in the lexical entry. in contrast to evokes that links to a lexical concept, ontolex:reference links to an ontological concept that represents a denotation of the lexical entry, here in the form of a wikidata entry. listing 4 displays the representation of the synset associated with both the lexical entry entry_w1 and the sense_w1_1-n. there we can also see that this lexical concept (synset) is also "evoked" by other entries/senses. for example by the entries for "kernfission" or "atomspaltung", which are synonyms of "kernspaltung". the lexinfo:hypernym property provides information on the semantic relation this synset has to another synset. finally, in listing 5 we display the entries for the components of the compound word "kernspaltung". those components point to the lexical entries they are related to (the entry :entry_w23527 is for example the one corresponding to the noun "spaltung" (split, fission, separation, cleavage, etc.), which has again its own senses and associated synsets. we can here disambiguate the meaning of "spaltung" as used in the compound, as being the one of "fission". and the whole compound can then be considered as a hyponym of the synset for "fission". listing 5: the two components of entry for "kernspaltung" in listing 1, we can see the information on the sequence those components have in this entry. for sure, those component entries can be re-used separately for other compounds, such as for "atomspaltung". thereby, we can collect all the corresponding meanings of a word, also when they are used in compounds and in dependency on their relative position in the compounds. a detailed representation of the decomposition module of ontolex-lemon is shown in figure 2 . in this section we described the current state of the ontolex-lemon representation of filtered or cleaned data we can find in odenet. furthermore, we touched upon the possible use of ontolex-lemon as a bridge between wordnetlike resources and full lexical descriptions, here exemplified with the case of german compound nouns. in the next section we address the issue on representing sense variants in dependency of the singular or plural inflection of an entry. ## added-values of the use of lemon-ontolex for representing wordnets as stated in the preceding section, we see the use of ontolex-lemon for representing wordnets as a chance to not only port information from one format to another, but also as an opportunity to extend the coverage of wordnet descriptions to more complex lexical phenomena, beyond lemma and pos considerations. one case we have been investigating concerns the different synsets that are attributed in pwn to the singular and to the plural forms of one word. when searching for a word in the pwn interface foot_8 , all potential synsets for this word are returned. while it is possible to actively search for plural forms of a noun, in a vast majority of cases the interface returns results for its uninflected counterpart because it lemmatizes the queried word. in cases of complementary plural entries, wordnet displays augmented lists of synsets: those associated with the singular, e.g. people, and those associated with the plural, e.g. peoples. all senses for this example are displayed in listing 6. this differentiation of grammatical number in the representation of synsets and associated meanings intuitively suggests that plural and singular forms do not share all meanings. regular cases, such as car returns no additional synsets and senses for its inflected form cars. thus, it can be assumed that the change of grammatical number does not cause any sense variant in those cases. this means, in turn, that it can be assumed that the availability of additional senses indicates that semantic differences exist between the inflectional forms. we also observe that querying a plural in word-net always results in the listing of all singular senses of a word and, where available, senses specific to the plural. however, this rigorous listing of singular senses also applies to plural nouns that share no sense with their singular counterpart. for instance, querying the pants khakis would result in a listing of all senses related to khaki and that of the plural. in case a sense exists only for a plural form, it would be desirable for the system to return only the corresponding synset. mixed cases exist for this phenomena, the ones where singular and plural share senses and those where senses are specific the singular or plural form. we showcase this behavior with the word pair letter-letters. while several senses can be associated with both the singular and the plural form of the lexical entry letter, the literary culture sense can be associated only with the plural form. on the other hand, the sense of literal interpretation (e.g. in the case of law texts that are interpreted by the letter) is generally assigned to the singular form. in the following listings, we show, in a simplified manner, the way this complex information can be encoded in ontolex-lemon. listing 7 displays the lexical entry for letter. it is stated that two forms are associated with this noun: a singular (the canonicalform) and a plural (the otherform) form. in this simplified entry, we link only to one sense: the one of an exchange between two parties (see listing 10). the next listing is about the shared sense associated with the lexical entry. as there is a wikidata entry for the type of entity this sense can refer to, we make use of the ontolex:reference property in order to link to this data source. listing 11 introduces the additional lexical entry for the plural form of letter that has a specific meaning that cannot be associated with its singular form. therefore we link this entry only to the plural instance of the class form and to the specific sense encoded in listing 12, where we additionally formulate the constraint that the usage of this sense is restricted to the plural form letters. in fact the use of the ontolex:usage property could suffice in order to mark that a sense is restricted to a particular inflectional form of an entry, as exemplified below in listing 13 for the sense of the literal interpretation, without the need to introduce a new lexical entry. ontolex-lemon in this case seems to be able to provide for a representation that would support morpho-semantic phenomena. as part of our future work, a possibility to associate senses to forms as well as lexical entries in the ontolex-lemon model is investigated. ## conclusion we described our current work consisting in porting a recently developed german wordnet compliant lexical resource, odenet, to ontolex-lemon, in order to support its publication in the linguistic linked open data cloud. while processing those data, we noticed that ontolex-lemon can be used for bridging the wordnet type of lexical resources to a full description of lexical entries, leading possibly to an extension of the coverage of wordnets beyond the consideration of lemmas and pos information. we documented this with the example of the representation of components of german compounds and the distinct senses that can exist between certain singular and plural forms of english words. in terms of future work, other types of full lexical descriptions will be modeled in ontolex-lemon and associated with the presented resources. furthermore, this type of modeling allows for cross-linking to other german wordnets in the llod, such as lemonuby. this crosslinking effort intends to finally interlink multilingual wordnets in a linked data-based format and its rich potential for full lexical descriptions.