| { |
| "paper_id": "C69-2001", |
| "header": { |
| "generated_with": "S2ORC 1.0.0", |
| "date_generated": "2023-01-19T12:32:42.257939Z" |
| }, |
| "title": "ON THE PRESERVATION OF CONTEXT-FREE LANGUAGES IN A LEVEL-BASED SYSTEM", |
| "authors": [ |
| { |
| "first": "Jacob", |
| "middle": [], |
| "last": "Mey", |
| "suffix": "", |
| "affiliation": { |
| "laboratory": "", |
| "institution": "The University of Texas at Austin", |
| "location": {} |
| }, |
| "email": "" |
| } |
| ], |
| "year": "", |
| "venue": null, |
| "identifiers": {}, |
| "abstract": "In this paper, a recently proposed level-oriented model for machine analysis and synthesis of natural languages is investigated\u00b0 Claims concerning the preservation of context-free (CF) languages in such", |
| "pdf_parse": { |
| "paper_id": "C69-2001", |
| "_pdf_hash": "", |
| "abstract": [ |
| { |
| "text": "In this paper, a recently proposed level-oriented model for machine analysis and synthesis of natural languages is investigated\u00b0 Claims concerning the preservation of context-free (CF) languages in such", |
| "cite_spans": [], |
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| "eq_spans": [], |
| "section": "Abstract", |
| "sec_num": null |
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| "body_text": [ |
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| "text": "Central to this model is the notion of a \"multi-level\" or \"stratified\" grammar. The generation of a sentence at the highest (\"deepest\") level of the grammar proceeds by a set of context-free (CF) rules| the output of these rules is then transduced to lower levels by a series of pushdown store automata. The output of the final transduction is some \"surface\" representation of the sentence to be generated. there is a CS based TG that will generate it (9:4.1).", |
| "cite_spans": [], |
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| "text": "As the authors point out (ibido:33), it is imperative to set out and find conditions under which a TG will generate only recurslve languages. One such condition would be to restrict the base of a TG to be CF; however t t-nls will still not guarantee recurslvlty (9:4.2)\u00b0 This is preclsely the problem that the Czechoslovak I workers will have to solve in order to make thelr system vlable and to valldate their claims.", |
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| "sec_num": null |
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| { |
| "text": "Mey 10", |
| "cite_spans": [], |
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| "section": "", |
| "sec_num": null |
| } |
| ], |
| "back_matter": [], |
| "bib_entries": { |
| "BIBREF0": { |
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| "urls": [], |
| "raw_text": "Po Sgall, Generativn~ Po_o~Jazvka a ~esk~ Dek~inace, Prague 1967.", |
| "links": null |
| }, |
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| "urls": [], |
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| }, |
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| "urls": [], |
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| }, |
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| "title": "A Note on Preservation of Languages by Transducers", |
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| "raw_text": "S\u00ae Ginsburg & G. Rose, \"A Note on Preservation of Languages by Transducers\", info Contr. 12(1968):549-552\u00b0 (See also (I0), below)", |
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| "volume": "9", |
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| "raw_text": "S. Ginsburg & G. Rose, \"Preservation of Languages by Transducers\", In~fo Contr. 9(1966): 153-176. (See also (I0), below) Mey 11", |
| "links": null |
| }, |
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| }, |
| "BIBREF9": { |
| "ref_id": "b9", |
| "title": "This review contains a succinct statement of the contents of (6) and (7), as well as a historical footnote on the authorship of the maln theorem of Section 2 of (7) and Its revised form as presented", |
| "authors": [], |
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| "raw_text": "J. Mey~ Review of Glnsburg & Rose (6) 9 Combo Re..~v. lO(May 1969), in press. (This review contains a succinct statement of the contents of (6) and (7), as well as a historical footnote on the author- ship of the maln theorem of Section 2 of (7) and Its revised form as pre- sented In (6)).", |
| "links": null |
| } |
| }, |
| "ref_entries": { |
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| "num": null, |
| "text": "system are examined and shown to be unjustified. Furthermorep it is ~hown that even a revised version of the mode1 (incorporating some recent discoveries) will not be CF-preservlngo Finallyp some theoretical implications of these findings are explored: in par-tlcular~ claims of greater naturalness and the question of recurslvltyo \u00ae Over the yearsp and especlally during the pr~waratlon of this paper t I have had the pleasure of many enlightening discussions with Stani%y Peters, for which I am glad to thank hlm. Mey la Dans ce travail, on envisage un module r~cent de synth~se et d'analyse automatiques de l angues naturelles, oriente vers la notion de \"nlveau\" linguistlque. On examine un postulat selon lequel !es langages contextfree seraient stables dans un tel syst~me~ Cette notion s'avere Incorrecte~ De plus~ on montre comment m~me une ~erslon modifi~e du module, incorporan~ certalnes d~couvertes r~centes~ ne conserve pas le caract~re context-free de ces langages. Enfin, on explore l'importance theorique de ces resultats~ en particulier, on examine l'avantage suppose d'un modele dit plus \"naturel\", et la question de la recurslvite des langues naturelles. Pendant les ann~es, et surtout pendant la r~daction de ce travail, J'ai pu profiter de nombreuses conversations illuminantes avec Stanley Peters, puur lesquelles Je tiens a lul exprimer ma reconnaissance. Mey 2 1. Level-Oriented Systems in Computational Linguistics A new model for computational linguistic perfor-m~,~ has recently been proposed by the Czechoslovak group of workers at Charles Unlverslty~ Prague~ under the direction of Po Sgall (lp 2). This model has Slgnificant theoretlcal impllcations, since it offers an alternative to transformationally based solutions of the problems encountered in automatic syntactic analysis t and consequently~ to the transformatlona~ model itselfo In particular, the new model claims to compete favorably with the transformational one ~> generative power and structural characterlzatio~; .J sentences.", |
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| "text": "CF-Preservatlon under TransductionThe whole system des\u00a3ribed in the preceding section is said to be \"weakly equivalent to a CF phrase structure grammar\" (1:148, 221). This assertion is claimed to derive from theorems formulated by N. Chomsky and Ro J. Evey(3; 4), which maintain that the output of a pushdown store transducer (pdt) is equivalent to the set of CF languages (i:109; 2:2.2.5).However, the original theorems about this equivalence concerned pushdown store acceptors (pda), not transducers (pdt as defined by Ginsburg (5:102)); for pdt (as used by Sgall and his group) t CF-preservation is known not to obtain in general (5:104).The condition under which pdt will preserve CF languages is stated by Ginsburg in Th. 3.5.1.(5:104)'. given a pda M t a pdt S, L -T(M), S(L) is CF Iff M and S are associated, i.e. S is obtained by adding outputs to M, the pda that accepts L. For the case of the Czechoslovak \"battery\" of transducers, this condition could actually be fulfilled, although the authors never say so, explicitly\u00b0 I am Mey 4 referring to their extra condition on the system, as formulated in \"the existence of inverse automata for the single levels of the pushdown part [of the grammar, JM] \" (1:146). Hence the impreciseness of the Czechoslovak proposal may amount to no more than a matter of incomplete formulation. In a recent article, however,-Ginsburg and Rose (6) have shown that the earlier theorem on which they based CF-preservatlon conditions for pdt is false\u00b0 (And~ by the same tokenp so are the original theorem of Evey's (4:2\u00b06\u00b06) and an earller theorem by Ginsburg and Rose (7:3.2)). According to the 1968 revised version of the latter ~heorem by Ginsburg and Rose (6:3\u00ae 2~)~ CF~p~eserva~ion is made dependent upon an addl-tlona~ condition on the pdt~ n~m~y~ that o~ ~est~Icting its ou~pu~ %o '~hese strings that are produced by the device when it ends up in an accepting (oz final) state, In all other cases~ the language generated by the pdt will not be equivalent to the set of CF languages, but simply constitute a ~ecurslvely enumer-Consequences for the Prague System For the case under consideration, the new insight referred to in the preceding section has two slgnlflcant consequences: first, the worries of the Czechoslovak group to ensure CF preservation may well have been in vainf unless the new condition can be incorporated into their systemo Otherwiset a device that is practically equlvalent to a Turlng machine is not very exciting to work with in computatlonal linguistic theory or its implementation. Second~ one of the advantages inherent in the use of CF-preservlng transducers is the guaranteed existence of a whole bevy of working recognition routines (e.g., the Cocke-Roblnson algorithm, the parser developed by Kay I or the predictive analyzer by Kuno, etc.) This advantage becomes illusory if the pdt battery produces a recursively enumerable languaget ioe. one that cannot be guaranteed to be recognized by a CFrecognition routine. If we think of the Czechoslovak system as part of a Mey 6 machine translation proposal, where the route from source to \"Interllngua\" consists of essentially the same flow (but in the opposite direction) as that from \"interllngua\" to target language, it appears that the consequences of an incorporation of the Ginsburg-Rose adjustment are far-reaching. Such an incorporation could take place in two ways: either one could check the output of a particular (~-th) device, transducing from a higher (~-th) to a lower (~ -lth) level, or from a lower (~-th) to a higher (~ \u00f7 lth) level~ to see whether or not this output corresponds to an accepting state (where \"higher\" and'~oweE\" are understood to refer to deeper and more superficial structures respectively); oft alternatively, a built-ln checking device could prevent output from being generated unless the transducer reached an accepting state after reading the input string. So far t Sgall and his group have not suggested ways to handle this problem. Quite another matter is that the Prague group's p\u00a3oposal to let the output of the ~-th de,ice be \u2022 proper s~bset of the input of the ~ + Ith~ ~ -Ith Mey 7 transducer respectively, does not seem to be fruitful, or even feasibleo Naturallyp the first question that arises is: what about the remaining input, where does it all come from? It is certainly true, as the authors remark (2:2o2o5)~ that \"the output language of the whole description is not necessarily context-free\"o AS shown above, it simply never is under the given conditions. Hence the reason given in the rest of the quote is trivial: \"since \u00b0.. it is only a proper subaet of the context-free languages of the last pushdown transducer\" (ibld.)o While it is always possible to tame a prope~ subset of a CF language and obtain a language tha% i~ not C2~ or maybe not even reguiar~ that cleaxly i~ not the point here. The authors in~nd their output language to be CF~ for reasons llk~ the ones mentioned above\u00b0 As long as it can be shown (as I have done here) that the system in no case is (even weakly) equivalent to a CF grammar~ the question of the restrictions on the input to the subsequent transducers is t of course, irrelevant to the CF character of the system as a whole.", |
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| "text": "consolation to the Prague ~oup that TG itself is in the same boat, theoretically speaking. In an important recent study~ Peters and Ritchle have demonstrated that a context-sensltlve (CS) based transformational grammar~ unless restricted in some respecta~ generates a recurslvely enumerable language~ and conversely, for any recurslvely enumerable language", |
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| "content": "<table><tr><td>Mey 8 Mey 9</td></tr><tr><td>4. Some theoretical implications respect to sentence recognition? It has been kno~</td></tr><tr><td>for a long time that there is no way of establlshing</td></tr><tr><td>The proponents of the Prague model have repeatedly a universal automatic recognition procedure for a</td></tr><tr><td>asserted that their system has certain advantages tr~sformatlonal grammar that is not in some ways</td></tr><tr><td>over other models of linguistic performance and, in restricted\u00b0 This was precisely the Prague group's</td></tr><tr><td>particular t that their grammar is superior (or at motivation for proposing their system as a (superior)</td></tr><tr><td>least equivalent) to transformational grammar. alternatlve to TGo Now that their clalms have been</td></tr><tr><td>The claim that the level-based model is superior to de-substantlated on theoretlcal grounds, it may seem</td></tr><tr><td>others because of easy CF-recognitlon has been. dispro-like a meagre</td></tr><tr><td>ved in the preceding sections. Another claim, that of</td></tr><tr><td>greater naturalness inherent in the level-orlented</td></tr><tr><td>model, is also often made (sometimes implicitly by</td></tr><tr><td>reference to the model's stance in tlme-honored lin-</td></tr><tr><td>guistic tradition). It should be observed t however~</td></tr><tr><td>that such a claim does not concern the formal charac-</td></tr><tr><td>ter of any system. As Chomsky has pointed out (in his</td></tr><tr><td>discussion of Fillmore's case theory (8:14-16)) I it</td></tr><tr><td>is vacuous to discuss different formal systems in</td></tr><tr><td>terms of which is the more \"direct\" representation</td></tr><tr><td>of natural language; unless a formal system is inter-</td></tr><tr><td>preted 0 it slmply cannot be compared to another one</td></tr><tr><td>for \"~Lrectneee\" of expression.</td></tr><tr><td>But how about transformational grammar itself with</td></tr></table>" |
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