Encoding UNL Expressions: Some Problems and Proposals

Encoding UNL Expressions: Some Problems and Proposals Igor Boguslavsky UNL Russia bogus@iitp.ru

Generals • UNL: an interlingua or not? • No ambiguity • UNL vs. natural language • “at least as powerful as any NL”? • semantics vs. KB • Correct UNL vs. adequate UNL

Adequacy conditions An adequate UNL expression should: • preserve the meaning of the source text; • be convenient for prospective applications, including deconversion.

Possibility of inverse generation? Necessary but insufficient: • invitation of the president • mod(invitation, president)? • the president invites somebody • somebody invites the president • Russian: He received the shower (= took the shower)

Tentative procedure How to develop a definite and common view on what UNL expressions are adequate? • UNL from LCs to UNLC • Comments from LCs to UNLC • Feedback from UNLC to LCs • Update of UNL by LCs

Universal Words • Headwords • Restrictions • Attributes

Headwords • Multi-word UWs • Support verbs

Multi-word UWs They should be avoided, if the their meaning is representable as a combination of meanings of words they are composed of: • UW to be avoided: «Ministery of foreign affairs» • UW to be preferred: mod(ministery.@entry, affair.@pl) mod(affair.@pl,foreign)

Why so? If any free word combination can be made a UW, one can never hope that other partners will have matching UWs in their dictionaries.

Appropriate multi-word UWs • Non-compositional phrases: • «look for(agt>thing,obj>thing)» • «look like(aoj>thing,obj>thing)»

A convenient compromise • To account for the fact that a phrase is considered as denoting a single concept, the UNL expression can be enclosed in a scope: mod:01(ministery.@entry, affair.@pl) mod:01(affair.@pl,foreign)

Another possibility (Ch. Boitet) • Postulate one UW having the internal structure: «mod(ministery,affair.@pl)_mod(affair.@pl, foreign)»

Restrictions in UW/KB • Semantic function • Knowledge Base function • Argument frame function

(1) Semantic function • Restricting the meaning - needed, in particular, to ensure • disambiguation of the head word • selection of the translation equivalent

(2) KB function • Locating the UW in the KB - needed, in particular, to ensure • choice of a nearest UW in the case the direct equivalent is absent in the UW dictionary (replacement ability) • semantic inference

(3) Agrument frame function • Presenting the argument frame.

Correlation between the semantic and the KB functions Semantically- and KB-oriented restrictions do not necessarily coincide: • semantic restriction: book(icl>thing) • KB restriction: titmouse{(icl>bird)}

How to select semantic restrictions • September{(icl>month>date)} • answer(icl>do) (for cases like answer questions) – answer(icl>be) (for cases like answer expectations) – answer(icl>thing) (for cases like know the answer) • Ru: zhenit’sja – marry(agt>male), vyxodit’ zamuzh – marry(agt>female).

VERY IMPORTANT! • Semantic restrictions should effectively distinguish the meaning we restrict from all other relevant meanings of the same English headword. • They should NOT be equally applicable to more than one meaning. • They should be easily understandable.

Example • operator - all the meanings denote a thing • WRONG (in the sense ‘inadequate’): • operator(icl>thing) • CORRECT • operator(icl>human) • operator(icl>abstract thing)

Relations useful for disambiguation • icl • iof • equ • ant (disappeared from the specs?) • poor(icl>bad): poor quality • ??? poor(icl>having little money) • “having little money” is a bad UW • poor(ant>rich): poor people

Needs to be emphasized again • UNL News1: build global knowledge • build(agt>thing,obj>thing) • Does not differentiate between different meanings of the headword: • build a railway (a house): build(agt>thing,obj>concrete thing) • build plans (knowledge): build(agt>thing,obj>abstractthing)

KB function • UW: September • MD: September{(icl>month>date)}

What remains unclear-1: KB  semantics. Links between related concepts of different semantic categories are missing. There is no way to express the relationship between “dance” (as a verbal concept) and “dance” (as a nominal concept)), “government” and “governmental”, etc. • dance({icl>do(}agt>person{)}) • dance(icl>action{>event})

What remains unclear-2: • Status of UWs within the restrictions: • propose(agt>thing,gol>thing,obj>thing) • They proposed to the president that a special committee should be set up • «set up» does not fall under «thing». But where then?

What is an argument-1? • A is an argument of L --> A is integral to the meaning of L.

What is an argument-2? • A is semantically obligatory: L cannot be semantically defined without A being mentioned. • A is not always syntactically obligatory: it can remain unmentioned in a sentence.

Example: buy • buy has 4 arguments: a buyer, an object, a seller, the money paid. • All of them are semantically obligatory: “buying” cannot exist without any of them. • None of them is syntactically obligatory: • I bought a book (the seller and the money are not mentioned). • To buy is more pleasant than to sell (no arguments are mentioned).

Semantic roles vs. predicate-argument relations • UNL does not mark predicate-argument relations in a systematic way. • Assumption: arguments can be reliably identified based on their semantic role.

It does not work. Why? • Too many «difficult» cases. Only a part of semantic relations between the words can be reliably interpreted in terms of semantic roles. • Too many mismatches. Assignment of semantic roles cannot be done in a consistent way (especially in the UNL multi-lingual and multi-cultural environment).

The reason: Numerous mismatches in the representation of the same or similar phenomena are rooted in the fundamental impossibility of a consistent interpretation of ALL argument relations in terms of a fixed SMALL number of semantic roles.

Examples • Nothing (obj) prevents the members (ben) from discussing (gol) this problem • why beneficiary (ben)? • why finite state (gol)? • protect nature from pollution (?) • familiarize students with India (?)

Difference between arguments and non-arguments-1 • Any nominal concept can have a purpose, e.g. • a stone for driving nails • Therefore {pur>uw} is assigned to UW «thing» and is inherited by all UWs lying below.

Difference between arguments and non-arguments-2 • Purpose is NOT an argument of “stone”: a stone has no obligatory conceptual link with the purpose. • Purpose IS an argument of “method”: a method cannot exist without a purpose. • a method for calculating taxes

Another example: borrow • X borrows Y from Z for W = • Z owns Y, • X makes Z to give him Y, • X promises Z to give Y back after W expires • borrow cannot exist without 4 participants: agent, object, source, duration

Difference between arguments and non-arguments-3 • Each action has a certain duration. Therefore {dur>time} is assigned to UW «do» and is inherited by all UWs lying below. • Besides this, borrow has a semantic argument with the role ‘duration’

Argument vs. non-argument (1) John borrowed money for 3 years • Argument W: term of the loan. John promised to return money after 3 years (2) John has been borrowing money for 3 years • Non-argument: the situation ‘John is borrowing money’ lasted for three years (the term of each loan is not specified)

Why important? - For semantic processing • (1) can answer the question on the terms of the loan - (2) cannot do so. • mod(invitation, president) • the president invites somebody (arg. 1) • somebody invites the president (arg. 2) • the invitation has an unspecified connection with the president (non-arg.) • NB: the specs do not allow to draw this distinction!

Why important? - For deconversion • The arguments and non-arguments are very often encoded differently: • dur: Ru «borrow on 3 years» vs. during 3 years • rsn: afraid of darkness, tremble with fear not: *afraid because of darkness • scn:In[scn]this country the relations between the nations are based on[scn-arg] mutual respect

How to differentiate? The distinction between the arguments and the non-arguments should be drawn both in the UWs and in UNL expressions.

Proposal for UWs • Restrictions corresponding to arguments should be systematically and exhaustingly represented in KB. • They can either be included into the UW, or be inherited from upper concepts.

Proposal for UWs • They should be formally opposed to non-argument restrictions. • One of the ways: capitalization. • thing{(and>thing,…,pur>uw,…)} • method(icl>abstract thing,Pur>uw) • do{and>do,…,dur>period,…)} • borrow(icl>do,…Dur>period) • Another possibility: • dur vs. dur.@A

Proposal for UNL expressions • Mark argument relations (Ch. Boitet): • @A, @B, @C… • relation for (1): • dur.@A(borrow, year) • relation for (2): • dur(borrow, year)

Sample UW dictionary entry • Current UW: responsible(aoj>thing,obj>thing) • It is proposed to introduce a comment: responsible (Aoj>thing,Obj>thing,Gol>*) ;he (aoj) is responsible to me (gol) for his actions (obj) (example) ;IB_Ru, 29/11/02 (author and date)

Comments in MD • Not only for illustrating argument frames, but also for clarifying concepts. • Specs modification is needed.

Attributes • Dictionary of attributes (explanation of the attribute, examples) • Procedure for introduction new attributes should be set up.

Issues concerning KB • Adjectival concepts

Adjectival concepts: mod vs. aoj • Two major classes of adjectives: • (aoj>thing) vs. (mod<thing) • Specs: “For an adjectival concept, (aoj>thing) or (mod<thing) should be attached to the Basic UW. (aoj>thing) is for expressing a predicative concept, whereas (mod<thing) is for expressing restrictive concept”.

We should distinguish between: (a) a syntactic property: whether the adjective is used predicatively (Greeks are wise) or attributively (the wise Greeks); (b) a semantic property: what does the adjective mean when used attributively: • restriction; • qualification. Only (b) should interest us.

Restrictive vs. non-restrictive • Wise Greeks diluted wine with water • restrictive: Those Greeks who were wise diluted wine with water. Silly ones didn’t. • non-restrictive (qualificative): Greeks were wise. They diluted wine with water. • Non-attributive (predicative) adjective does not restrict the noun: • Greeks are wise.

Encoding UNL Expressions: Some Problems and Proposals