UNL2010

Revision as of 13:04, 21 February 2010

These guidelines were built to normalize and standardize UNLization strategies and to guide the development of the natural language generation grammars (which would benefit from these standards as a sort of NL-to-UNL transfer grammar). They should not be taken as the UNL Specifications themselves, as long as 1) they are rather experimental and tentative; 2) they differ, in several points, from the current version of the Specifications; 3) they do not follow some of the existing UNLization policies; and 4) they have not been approved yet by the UNL Community.

PREMISES

These guidelines are derived from three main premises:

Information conveyed by natural language utterances can be represented a natural language independent hypergraph structure.

Texts can be treated as a set of semantic nodes interlinked by semantic relations and modified by semantic attributes. Nodes can be either simple (UWs) or complex (SCOPES, i.e., subgraphs, such as clauses).

The UNL representation is an intrepretation rather than a translation of a given text. 

The main goal of the UNLization process is to represent the knowledge structure of the source text, which should be detached from its verbal structure. This means that the UNL representation should not be committed to replicate the lexical and the syntactic choices of the original, but should focus in representing, in a language-independent and non-ambiguous format, one of its possible readings, preferably the most conventional one.

The UNL representation should be as semantically complete as possible. 

This means that, whenever possible, all the semantic valencies of the original text should be saturated, including anaphora, ellipses, presuppositions and implicatures. Pronouns and pro-forms, for instance, are expected to be replaced by their antecedents, and should not be represented in UNL, except in case of exophoric reference (indefinite pronouns, interrogative pronouns and personal pronouns that are not coindexed to any existing antecedent).

UNL EXPRESSION

For the time being, the network macrostructure has not been addressed, but it seems clear that the relation “nxt”, proposed by the UNL Centre to link sentences and paragraphs, is syntactic rather than semantic, and it is not appropriate for a network claimed to be mostly semantic. Some alternatives have been considered (especially Discourse Representation Theory - DRT, proposed by Hans Kamp; and Rhetorical Structure Theory - RST, proposed by William Mann and Sandra Thompson), but they are still under investigation.

The document structure is also subject to change, and it is likely to move to a XML schema, which is still under development. For the moment, the syntax defined by the UNLCenter has been kept.

THREE-LAYERED REPRESENTATION

The basic assumption of the UNL approach is that the meaning conveyed by natural language sentences can be formally represented through three different types of semantic units: UWs, attributes and relations. This three-layered representation model is the cornerstone of UNL and its most distinctive feature over other semantic networks, which normally proposes only two levels: edges and vertices. Nevertheless, it poses several problems to the UNLization as the distinction between what is supposed to be represented by each unit is not always clear. In order to avoid superposition and to facilitate the enconversion process, we have tried to clearly identify the scope of each unit using the following procedures:

• RELATIONS represent syntactic relations (subject, object, complement, adjunct) with their corresponding semantic value;
• UWs represent lexemes from open classes:
  • nouns, including proper nouns, abbreviations and acronyms;
  • adjectives;
  • full verbs;
  • adverbs (adjunts, conjuncts and disjuncts); and
  • numbers (to be always represented as Arabic numerals)
• ATTRIBUTES represent bound morphemes, closed classes and context-dependent information:
  • grammatical categories (gender, number, tense, aspect, mood, voice, etc)
  • determiners (articles and demonstratives);
  • adpositions (prepositions, postpositions and circumpositions);
  • auxiliary and quasi-auxiliary verbs (auxiliaries, modals, coverbs, preverbs);
  • interjections;
  • conjunctions;
  • adverbs (specifiers);
  • text structure (.@entry, .@topic, .@qfocus, .@emphasis, .@relative, etc);
  • speech acts (.@request, .@suggestion, .@offer, etc);
  • other context-dependent information (such as politeness, metaphor, irony, etc);

Pronouns and pro-forms are expected to be replaced by their antecedents and not to be represented in UNL, except in case of exophoric reference (indefinite pronouns, interrogative pronouns and personal pronouns that are not coindexed to any existing antecedent).

The main changes concerning the present UNL Specifications are the following:

RELATIONS

The set of relations is exactly the same as defined in the UNL 2005 Specifications.

ATTRIBUTES

The set of attributes has been substantially increased to represent information concerning grammatical categories, determiners, adpositions and conjunctions. The main additions are the following:

• gender: @male, @female
• degree of comparison: @more, @less, @equal, @most, @least
• demonstrative: @proximal, @medial, @distal
• preposition: @under, @below, @above, @after, @before, etc.
• conjunction: @before, @after, etc.
• relative (for the main entry of relative clauses): @relative

The decision to represent closed classes as attributes instead of UWs has led to a different way of representing several natural language phenomena:

this X

UNL Centre: mod(X, this)

These guidelines: X.@proximal

X is under Y

UNL Centre: plc(X, under), obj(under, Y)

These guidelines: plc(X, Y.@under)

bigger than Y

UNL centre: man(big, more), bas(big, Y)

These guidelines: bas(big.@more, Y)

etc.

Additionally, the following general principles were adopted:

• interjections, filled pauses, phatic expressions and short answers should be represented by the null UW (to be represented as "00") together with the attribute indicating the corresponding speech act (.@confirmation, .@surprise, etc).
• the attribute .@entry (mandatory in every scope, including the main one) should be placed at the left (source) side of at least one relation;
• the difference between mentioning and using a word (which is a quite frequent situation in a metalinguistic text such as Cratylus) should be represented by the attribute .@mention (which is not the same as "quotation");
• attributes should be used in alphabetical order (“.@entry.@past” instead of “.@past.@entry”).

UNIVERSAL WORDS

The set of Universal Words, i.e., the UNL Dictionary, has undergone the most radical change, as we have been using the UNLWN30, a set of UWs automatically extracted out of the WordNet30. In this dictionary, UWs correspond to sets of synonyms (synsets) of English, and may have several different headwords. They are represented as 9-digit strings with the following format:

<POS><WORDNETID>

where <POS> = {1,2,3,4}, being 1 = noun, 2 = verb, 3 = adjective and 4 = adverb;
and <WORDNETID> is the synset ID in the WN3.0.

SCOPES

In order to enhance the possibility of knowledge extraction out of the UNL document, we have restricted the use of scopes only to cases involving semantic ambiguity, such as:

• electric [light orchestra], with scope, i.e., a "light orchestra" that is electric; or
• electric light orchestra, without scope, i.e., an orchestra that is both "light" and "electric".

INVITATION

Finally, we ought to stress that the UNLization standards here presented are tentative and provisional, and they are subject to improvements and changes as soon as they were proved not to be the most adequate ones. In order to provide such enhancements, we would invite UNL Society members and other people interested in UNL to criticize them, to propose alternatives and to help us build an NL-to-UNL transfer grammar as comprehensive as possible.

BROWSE BY ALPHABETICAL ORDER

abbreviations acronyms active voice adjectives adjuncts adverbs apposition articles aspect auxiliary verbs capitalization cardinals common nouns comparative adjectives comparative adverbs comparative complement complex words compound words conditional conjunctions conjuncts contractions coordination copula degree demonstratives determiners digits disjuncts equations fractions gender imperative indicative intensifiers interjections interrogative modal verbs mood multiplicative numerals nominalization numbers numbers in names numbers in titles numerals object ordinals passive person personal pronouns possessive pronouns premodifiers prepositions pronouns proper nouns punctuation marks quantifiers reciprocal pronouns reflexive pronouns reflexive voice relative clause relative pronouns spelling subject subjunctive subordination superlative adjectives superlative adverbs superlative tense typographical symbols verb modification verb modifiers verbs vocative voice

BROWSE BY SUBJECT