Lexica
The UNL System contains three different types of lexical databases: dictionaries, knowledge bases and example bases.
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Background
The lexicon of UNL is formed by the set of permanent UW's, which are expected to represent concepts lexicalized in at least one language. UW's, however, are simply uniform concept identifiers, i.e., arbitrary addresses or names that do not convey, themselves, any information. The meaningfulness of a UW is defined in four different lexical databases, which organize the structure of concepts in two basic levels:
- Semantic features (monadic predicates), such as semantic class, lexical category, abstractness, polarity, cardinality, etc., are expected to describe the most generic distinctive units of meaning of each concept. They are closely related to the notions of "classeme" (Pottier, 1965), and of "semantic markers" or "classifiers" (Katz & Fodor, 1963). They are used to classify UW's into generic semantic categories that can be used inside the grammar. Semantic features are represented inside the UNL Dictionary.
- Semantic frames (dyadic predicates) represent a collection of facts that specifies or distinguishes (i.e., "defines") each concept. They represent interactions between concepts that can be either "necessary" or "typical". The set of necessary (essential) interactions constitutes the UNL Knowledge Base; the set of typical (essential and accidental) interactions constitutes the UNL Memory, which includes the UNL Knowledge Base. The difference between "necessary" and "typical" interactions is a matter of logic: an interaction between two concepts X and Y is considered to be "essential" if Y is a logical consequence of X, i.e., if X entails Y; and it is considered to be "typical", if it is simply recurring[1]. The necessary interactions are further analyzed in monotonic and non-monotonic: the former corresponds to the relations "is-a-kind-of" and "is-an-instance-of", which constitute the UNL Ontology, a tree hierarchical structure which is part of the UNL Knowledge Base.
All these interactions, either necessary or typical, are represented as UNL graphs, i.e., as a coherent (network) structure made of UW's, relations and attributes, and are used mostly for word sense disambiguation and for lexicalization (i.e., to fill in lexical gaps).
Dictionaries
Main article: Dictionary Specs
In the UNL System, a dictionary is a flat list of entries with their corresponding features. The dictionaries comply with the structure defined in the Dictionary Specs and must contain only tags defined in the Tagset. They are divided in three different categories:
- The UNL Dictionary, or simply UNLdic, is a list of UW's and their semantic (language-independent) markers. It is divided into three different nested lexical databases: the UNL Core Dictionary, the UNL Abridged Dictionary and the UNL Unabridged Dictionary. The UNL Core Dictionary brings permanent UW's which are supposed to be lexicalized in all languages; the UNL Abridged Dictionary brings permanent UW's which are lexicalized in at least two language families (and includes therefore the UNL Core Dictionary); the UNL Unabridged Dictionary, which contains the UNL Abridged Dictionary, brings the whole sent of permanent UW's (i.e., the concepts that are lexicalized in at least one language).
- The NL Dictionary, or simply NLdic, is a list of natural language entries with the corresponding morphological and syntactic (language-dependent) features.
- The UNL-NL Dictionary, or simply UNL-NLdic, is list of lexical mappings between UW's and natural language entries. The UNL-NL Dictionary is provided in two different formats: the generative, which is used normally in natural language generation, brings only base forms and the corresponding inflectional rules; the enumerative, which has been used in natural language analysis, brings all the word forms.
The UNL Dictionary and the NL Dictionary are monolingual databases, whose entries are interlinked in the UNL-NL Dictionary, which brings the mappings between UW's and natural language entries, whenever available[2]. These three dictionaries are normally made through the UNLarium in different steps and constitute the basic resource for UNLization and NLization.
Knowledge Bases
Main article: UNL Knowledge Base
The UNL Dictionary is simply a flat list of UW's and their corresponding classifiers (such as lexical category, semantic class, abstractness, cardinality, etc.). The UNL Dictionary does not contain any distinguisher, i.e., any information that can be used to differentiate a given UW from the others that belong to the same class. This information is provided in the UNL Knowledge Base, or UNLKB, which is a semantic network made of relations that are necessary to define UW's.
The UNL Knowledge Base is expected to represent the intension (the meaning) of UW's.
The UNL Knowledge Base contains the UNL Ontology, which is a part of the UNLKB where UW's are interconnected by the ontological relations of UNL, i.e., "is-a-kind-of" ("icl") and "is-an-instance-of" ("iof").
Example Bases
In the UNL System, there are two different types of example bases:
- The UNL Memory is a network of UW's that extends and complements the UNLKB. The difference is that the UNLKB, which is dictionary-based, contains only necessary relations between UW's, whereas the UNL Memory, which is corpus-based, brings any relations between UW's along with their frequency of occurrence. For instance, the idea that a "table" is "supported by one or more vertical legs" is not represented in the UNLKB because it is not supposed to be necessary (there are tables that are not supported by legs). This information, as the information that tables are normally round or square, that they are made of hard materials, etc., is repesented in the UNL Memory, which is expected to represent not only common sense knowledge about UW's, but all the possible instances of a given UW.
- The UNL-NL Memory is a list of frequent mappings between UNL and a given natural language. It is the UNLization (translation) memory. Differently from the UNL-NLdic, which involves only lexical mappings, the UNL-NL Memory involves any UNLization units, which may include several lexical units.
Notes
- ↑ Consider, for instance, the case of the UW corresponding to the concept conveyed by the English word "table" (= piece of furniture). The interaction between "table" and "furniture" is considered "necessary", because there is no table, in that sense, which is not a piece of furniture. However, the interaction between "table" and "leg" is considered "typical", because, although highly frequent, there can be tables without legs.
- ↑ Not all NL dictionary entries may be mapped onto UNL. Articles, prepositions, conjunctions and other particles do not have any correspondence in UNL.
