Lexical Realisation Unit

Revision as of 15:56, 12 January 2010

In the UNLarium framework, a Lexical Realisation Unit (or simply LRU) is any discrete, recurring and standardized unit of meaning of a given natural language. It can be a subword(a root, an affix), a simple word or a multiword expression (compounds, collocations, idioms). The set of LRUs constitutes the vocabulary or the lexicon of a language.

LRUs are standardized lexical realisations for concepts

The UNLarium is first and foremost a generation-driven framework, which has been developed mainly to provide resources for generating natural language texts out of UNL graphs. In that sense, UNLarium entries should correspond to the most likely realisations, in a given language, of a given concept. The expression “realisation" stands here for a mixture of wording and phrasing, i.e., the manner in which a concept is articulated in a given language. For instance, the concept “the natural satellite of the Earth” is realised, in English, by the word “Moon”; in French, by “lune”; in German, by “Mond”; in Russian, by “луна”; in Spanish, by “luna”; in Chinese, by 月; and so on. Your first task in the UNLarium is exactly to find out linguistic realisations for concepts, which will be always presented by their corresponding definition in English.

LRUs, however, are not simply linguistic realisations; they are lexical realisations. This means that LRUs should correspond to the units of the vocabulary of a language, i.e., to a lexeme. Let’s come back to our previous example. Apart from “Moon”, the concept “the natural satellite of the Earth” can be realised, in English, by the very expression “the natural satellite of the Earth”, which is indeed very frequent (2.130.000 occurrences in Google). This expression, however, is a “definition” rather than a “lexical realisation” for the concept, and should therefore not correspond to a LRU.

The differences between definitions and lexemes, or between “defining” and “naming” a concept, are fairly subjective, and are normally ascribed to the compositionality (or analyticity) of the candidate term: if the meaning of the compound can be reduced to the combination of the meaning of its components, it is said to be simply a definition; otherwise, i.e., if there is a sort of semantic surplus, a supplementary (or even complementary) sense added to the simple combination, the term is considered a lexeme. Consider, for instance, the case of “sweet and sour”. In this case, we have clearly a lexeme and therefore a LRU, because the phrase encompasses the sense of “sauce” that cannot be derived from the combination of the meanings of “sweet”, “and”, “sour”, even if, in the past, the expression was merely a conjunction of two simple adjectives. As a matter of fact, the process of converting definitions into lexical items is very common and one of the most frequent strategies employed to increase and improve the vocabulary of a language.

Finally, LRUs have also to be standardized lexical realisations for concepts. This means that candidate terms must have been already crystalized, which can be verified by their presence in dictionaries and glossaries, or by their rate of recurrence in the Web. If the structure of the candidate term is reasonably constant and convincingly recurring, it is consolidated and therefore a potential LRU; otherwise, it is not. But be careful: the frequency condition applies after the lexical (combinatorial) one. Notice that “the natural satellite of the Earth” is not a LRU, in spite of its 2.130.000 occurrences, whereas “sweet and sour”, with 1.710.000 occurrences, is a LRU.

LRUs are not words

A common misunderstanding in natural language description is that, due to our writing conventions, especially in the Western tradition, we tend to reduce the lexicon of a language to a list of “words”, which are normally understood as the smallest free forms, or the strings of alphabetic characters isolated by blank spaces. Unfortunately, it is not that simple. The vocabulary of a language is made not only of words, but of parts of words (roots, stems, affixes, particles) and of multiple-word expressions (compounds, collocations, idioms). In English, one of the most frequent lexical realisations for the concept “contrary of” is the prefix “un-“, which is a bound morpheme (i.e., a semantic unit that does not have an independent existence); in the same way, the concept “allow or plan for a certain possibility” is frequently realised by the phrasal verb “to take (sth) into account”, which is a complex structure that does not figure as a separate entry in most English dictionaries (it is normally listed inside the verb “to take”). So, it is important to understand that “lexical realisation unit”, here, means not only “words”, in the common sense, but any part of the vocabulary of a language, which includes words, subwords or multi-word expressions.

LRUs are not morphemes

In synthetic (inflected) languages, such as the Indo-European ones, a single concept may be realised by different lexical realisations in order to express different grammatical categories, such as number, gender, tense and case. The definition “pass from physical life and lose all bodily attributes and functions necessary to sustain life”, for instance, is realised, in English, by the forms "to die" (infinitive), "die" (present tense except 3rd person singular), "dies" (3rd person singular of the present tense), "dying" (gerund), "died" (past tense), "dead" (past participle), "will die" (future), etc.

The morphological analysis allows us to perceive that, from the perspective of realising a concept, the most interesting entity is the root. Particles (such as "to"), affixes (such as the inflectional suffixes "-s", "-ing", "-ed") and co-verbs (such as the auxiliary "will") convey notions that are not included in the concept definition and should be isolated in order to obtain the real LRU. This does not mean, however, that LRUs are roots. As realisations of definitions, LRUs may correspond either to single-rooted forms (such as “die”) or to multiple-root compounds (such as “kick the bucket” or “give up the ghost”). Additionally, and depending on the concept to be realised, LRUs may correspond to stems (aka as inflectional roots), which are combinations of roots and derivational affixes. The concept "experiencing or marked by or causing sadness or sorrow or discontent", for instance, is realised, in English, by "unhappy", which is a combination of the prefix "un-" and the root "happy". Finally, LRUs may even include inflectional affixes, such as in "glasses", which is the lexical realisation of "optical instrument consisting of a pair of lenses for correcting defective vision". The form "glasses" contain a root "glass" and an inflectional suffix "-es", which expresses plural. It is quite difficult, thus, to associate the notion of LRU to that of a morpheme, since LRUs may be either mono-morphemic or pluri-morphemic structures. In any case, we normally consider that derivational affixes (such as "un-", "re-", "-er" and "-ness" in realisations such as "unhappy", "rewrite", "writer" and "happinesss") form new LRUs, whereas inflectional affixes form only different realisations of the same LRU, which should be understood, then, as a class, rather than as single element.

Apart form inflections, LRUs may vary in many other different directions: spelling (such as in the allomorphs “die” and “dy” above), discontinuity (as in “take […] into account”) and order (as in German separable verbs such as “angekommen”, which becomes “kommen” […] “an” in finite realisations). These variants are said to be simply instances of the same LRU, even in case of radical changes (such as the forms of the irregular verb "to be" in English: “be”, “am”, “are”, “was”, etc). As we will see later, the possible variations of a given LRU will be informed through special rules to be created inside the very LRU.

How to express a LRU

To assure readability and to allow the reference to all instances of the same LRU, the LRU is represented, in the UNLarium, through a lemma, i.e., a canonical (citation) form, which is the entry form normally given in ordinary dictionaries and glossaries. The lemma is the form of the singular, for nouns; of the masculine singular, for adjectives; and the infinitive, for verbs. The lemma should follow the spelling and the capitalization rules of the target language. In English, for instance, only proper names should bring an initial upper case, whereas in German all nouns should be written this way.

Lexicalisation divergences

As languages have different lexicalisation processes, a single definition may correspond to several different LRUs, which are said to be synonyms. The definition “pass from physical life and lose all bodily attributes and functions necessary to sustain life”, for instance, may be realised in English by several different LRUs: “die”, “croak”, “decease”, “drop dead”, “buy the farm”, “cash in one's chips”, “give-up the ghost”, “kick the bucket”, “pass away”, “perish”, “snuff it”, “pop off”, “expire”, “conk”, “exit”, “choke”, “go”, “pass”, etc. In such cases, all realisations should be informed in the UNLarium.

There are cases, however, in which the definition cannot be lexically realised [by a single lexical unit] in the target language. This happens in two situations:

• When the concept is underspecified, i.e., too broad (or vague) to be realised. The concept of “red entity”, for instance, may be coextensive with several different English LRUs (“blood”, “cherry”, “ruby”, “ketchup”, “Spiderman”, etc), but these are rather subordinate terms (or hyponyms), in the sense they only include and partly match the intended sense. And the expression “red entity” itself is too compositional and too occasional to be considered already lexicalized (Google brings only 8,040 occurrences for this bigram).
• When the concept is overspecified, i.e., too narrow (or specific) to be realised. Consider, for instance, the definition “a person who is ready to forgive any transgression a first time and then to tolerate it for a second time, but never for a third time”. This definition does not lead to any LRU in English, French or Russian, even though it corresponds to a single word (“ilunga”) in Tshiluba, a language spoken in the Republic of Congo. We may obviously express the concept in any language, but we have to do it through a periphrasis (as we have done for English) or through a superordinate term (or hypernym), such as “forgiver”, “excuser”, “pardoner”, which are again fairly accurate.

In both cases, there will be no realisation to be informed, and it is important to indicate, in the UNLarium, that the concept has not been lexicalized yet, which means that it can be expressed in the target language only by means of definitions (periphrases) and other semantically related (and inaccurate) lexical units (such as hyponyms or hypernyms).

Examples