{ "paper_id": "P97-1014", "header": { "generated_with": "S2ORC 1.0.0", "date_generated": "2023-01-19T09:15:22.847186Z" }, "title": "Centering in-the-Large: Computing Referential Discourse Segments", "authors": [ { "first": "Udo", "middle": [], "last": "Hahn", "suffix": "", "affiliation": { "laboratory": "Computational Linguistics Research Group", "institution": "Freiburg University", "location": { "addrLine": "Werthmannplatz 1", "postCode": "D-79085", "settlement": "Freiburg", "country": "Germany" } }, "email": "" }, { "first": "Michael", "middle": [], "last": "Strube", "suffix": "", "affiliation": { "laboratory": "Computational Linguistics Research Group", "institution": "Freiburg University", "location": { "addrLine": "Werthmannplatz 1", "postCode": "D-79085", "settlement": "Freiburg", "country": "Germany" } }, "email": "" } ], "year": "", "venue": null, "identifiers": {}, "abstract": "We specify an algorithm that builds up a hierarchy of referential discourse segments from local centering data. The spatial extension and nesting of these discourse segments constrain the reachability of potential antecedents of an anaphoric expression beyond the local level of adjacent center pairs. Thus, the centering model is scaled up to the level of the global referential structure of discourse. An empirical evaluation of the algorithm is supplied.", "pdf_parse": { "paper_id": "P97-1014", "_pdf_hash": "", "abstract": [ { "text": "We specify an algorithm that builds up a hierarchy of referential discourse segments from local centering data. The spatial extension and nesting of these discourse segments constrain the reachability of potential antecedents of an anaphoric expression beyond the local level of adjacent center pairs. Thus, the centering model is scaled up to the level of the global referential structure of discourse. An empirical evaluation of the algorithm is supplied.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Abstract", "sec_num": null } ], "body_text": [ { "text": "The centering model (Grosz et al., 1995) has evolved as a major methodology for computational discourse analysis. It provides simple, yet powerful data structures, constraints and rules for the local coherence of discourse. As far as anaphora resolution is concerned, e.g., the model requires to consider those discourse entities as potential antecedents for anaphoric expressions in the current utterance Ui, which are available in the forward-looking centers of the immediately preceding utterance Ui-1. No constraints or rules are formulated, however, that account for anaphoric relationships which spread out over non-adjacent utterances. Hence, it is unclear how discourse elements which appear in utterances preceding utterance Ui-1 are taken into consideration as potential antecedents for anaphoric expressions in Ui.", "cite_spans": [ { "start": 20, "end": 40, "text": "(Grosz et al., 1995)", "ref_id": "BIBREF3" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "The extension of the search space for antecedents is by no means a trivial enterprise. A simple linear backward search of all preceding centering structures, e.g., may not only turn out to establish illegal references but also contradicts the cognitive principles underlying the limited attention constraint (Walker, 1996b) . The solution we propose starts from the observation that additional constraints on valid antecedents are placed by the global discourse structure previous utterances are embedded in. We want to emphasize from the beginning that our proposal considers only the referential properties underlying the global discourse structure. Accordingly, we define the extension of referential discourse segments (over several utterances) and a hierarchy of referential discourse segments (structuring the entire discourse). 1 The algorithmic procedure we propose for creating and managing such segments receives local centering data as input and generates a sort of superimposed index structure by which the reachability of potential antecedents, in particular those prior to the immediately preceding utterance, is made explicit. The adequacy of this definition is judged by the effects centered discourse segmentation has on the validity of anaphora resolution (cf. Section 5 for a discussion of evaluation results).", "cite_spans": [ { "start": 308, "end": 323, "text": "(Walker, 1996b)", "ref_id": "BIBREF20" } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "There have been only few attempts at dealing with the recognition and incorporation of discourse structure beyond the level of immediately adjacent utterances within the centering framework. Two recent studies deal with this topic in order to relate attentional and intentional structures on a larger scale of global discourse coherence. Passonneau (1996) proposes an algorithm for the generation of referring expressions and Walker (1996a) integrates centering into a cache model of attentional state. Both studies, among other things, deal with the supposition whether a correlation exists between particular centering transitions (which were first introduced by Brennan et al. (1987) ; cf. Table 1 ) and intentionbased discourse segments. In particular, the role of SHIFT-type transitions is examined from the perspective of whether they not only indicate a shift of the topic between two immediately successive utterances but also signal (intention-based) segment boundaries. The data in both studies reveal that only a weak correlation between the SHIFT transitions and segment boundaries can be observed. This finding precludes a reliable prediction of segment boundaries based on the occurrence of 1 Our notion of referential discourse segment should not be confounded with the intentional one originating from Grosz & Sidner (1986) , for reasons discussed in Section 2. SHIFTS and vice versa. In order to accommodate to these empirical results divergent solutions are proposed. Passonneau suggests that the centering data structures need to be modified appropriately, while Walker concludes that the local centering data should be left as they are and further be complemented by a cache mechanism. She thus intends to extend the scope of centering in accordance with cognitively plausible limits of the attentional span. Walker, finally, claims that the content of the cache, rather than the intentional discourse segment structure, determines the accessibility of discourse entities for anaphora resolution.", "cite_spans": [ { "start": 338, "end": 355, "text": "Passonneau (1996)", "ref_id": "BIBREF10" }, { "start": 426, "end": 440, "text": "Walker (1996a)", "ref_id": "BIBREF19" }, { "start": 665, "end": 686, "text": "Brennan et al. (1987)", "ref_id": "BIBREF0" }, { "start": 1318, "end": 1339, "text": "Grosz & Sidner (1986)", "ref_id": "BIBREF4" } ], "ref_spans": [ { "start": 693, "end": 700, "text": "Table 1", "ref_id": null } ], "eq_spans": [], "section": "Global Discourse Structure", "sec_num": "2" }, { "text": "c~(v.) = cdu.-~) c~(u.) # OR Cb(Vn-1) undef. Cb(Vn-1) Cb(Un) = CONTINUE (C) SMOOTH-SHIFT (SS) c~(u.) cb(u.) # RETAIN (R) ROUGH-SHIFT (RS)", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Global Discourse Structure", "sec_num": "2" }, { "text": "c~(u.) Table h Transition Types As a working hypothesis, for the purposes of anaphora resolution we subscribe to Walker's model, in particular to that part which casts doubt on the hypothesized dependency of the attentional from the intentional structure of discourse (Grosz & Sidner, 1986, p. 180) . We diverge from Walker (1996a) , however, in that we propose an alternative to the caching mechanism, which we consider to be methodologically more parsimonious and, at least, to be equally effective (for an elaboration of this claim, cf. Section 6).", "cite_spans": [ { "start": 268, "end": 298, "text": "(Grosz & Sidner, 1986, p. 180)", "ref_id": null }, { "start": 317, "end": 331, "text": "Walker (1996a)", "ref_id": "BIBREF19" } ], "ref_spans": [ { "start": 7, "end": 31, "text": "Table h Transition Types", "ref_id": null } ], "eq_spans": [], "section": "Global Discourse Structure", "sec_num": "2" }, { "text": "The proposed extension of the centering model builds on the methodological framework of functional centering (Strube & Hahn, 1996) . This is an approach to centering in which issues such as thematicity or topicality are already inherent. Its linguistic foundations relate the ranking of the forward-looking centers and the functional information structure of the utterances, a notion originally developed by Dane~ (1974) . Strube & Hahn (1996) use the centering data structures to redefine Dane~'s trichotomy between given information, theme and rheme in terms of the centering model. The Cb(Un), the most highly ranked element of C! (Un-1) realized in Un, corresponds to the element which represents the given information. The theme of Un is represented by the preferred center Cp (Un), the most highly ranked element of C! ( Un ). The theme/rheme hierarchy of Un corresponds to the ranking in the C! s. As a consequence, utterances without any anaphoric expression do not have any given elements and, therefore, no Cb. But independent of the use of anaphoric expressions, each utterance must have a theme and a C! as well. The identification of the preferred center with the theme implies that it is of major relevance for determining the thematic progression of a text. This is reflected in our reformulation of the two types of thematic progression (TP) which can be directly derived from centering data (the third one requires to refer to conceptual generalization hierarchies and is therefore beyond the scope of this paper, cf. Dane~ (1974) Table 2 visualizes the abstract schemata of TP patterns. In our example (cf. Table 8 in Section 4), U1 to Ua illustrate the constant theme, while U7 to U10 illustrate the linear thematization of rhemes. In the latter case, the theme changes in each utterance, from \"Handbuch\" (manual) via \"Inhaltsverzeichnis\" (table of contents) to \"Kapitel\" (chapter) etc. Each of the new themes are introduced in the immediately preceding utterance so that local coherence between these utterances is established. Daneg (1974) also allows for the combination and recursion of these basic patterns; this way the global thematic coherence of a text can be described by recurrence to these structural patterns. These principles allow for a major extension of the original centering algorithm. Given a reformulation of the TP constraints in centering terms, it is possible to determine referential segment boundaries and to arrange these segments in a nested, i.e., hierarchical manner on the basis of which reachability constraints for antecedents can be formulated. According to the segmentation strategy of our approach, the Cp of the end point (i.e., the last utterance) of a discourse segment provides the major theme of the whole segment, one which is particularly salient for anaphoric reference relations. Whenever a relevant new theme is established, however, it should reside in its own discourse segment, either embedded or in parallel to another one. Anaphora resolution can then be performed (a) with the forwardlooking centers of the linearly immediately preceding utterance, (b) with the forward-looking centers of the end point of the hierarchically immediately reachable discourse segment, and (c) with the preferred center of the end point of any hierarchically reachable discourse segment (for a formalization of this constraint, cf. Table 4 ).", "cite_spans": [ { "start": 109, "end": 130, "text": "(Strube & Hahn, 1996)", "ref_id": "BIBREF5" }, { "start": 408, "end": 420, "text": "Dane~ (1974)", "ref_id": "BIBREF2" }, { "start": 423, "end": 443, "text": "Strube & Hahn (1996)", "ref_id": "BIBREF5" }, { "start": 1535, "end": 1547, "text": "Dane~ (1974)", "ref_id": "BIBREF2" }, { "start": 2048, "end": 2060, "text": "Daneg (1974)", "ref_id": null } ], "ref_spans": [ { "start": 1548, "end": 1555, "text": "Table 2", "ref_id": "TABREF1" }, { "start": 1625, "end": 1632, "text": "Table 8", "ref_id": "TABREF10" }, { "start": 3383, "end": 3390, "text": "Table 4", "ref_id": "TABREF3" } ], "eq_spans": [], "section": "Global Discourse Structure", "sec_num": "2" }, { "text": "Prior to a discussion of the algorithmic procedure for hypothesizing discourse segments based on evidence from local centering data, we will introduce its basic building blocks. Let x denote the anaphoric expression under consideration, which occurs in utterance Ui associated with segment level s. The function Resolved(x, s, Us) (cf. Table 3 ) is evaluated in order to determine the proper antecedent ante for x. It consists of the evaluation of a teachability predicate for the antecedent on which we will concentrate here, and of the evaluation of the predicate lsAnaphorFor which contains the linguistic and conceptual constraints imposed on a (pro)nominal anaphor (viz. agreement, binding, and sortal constraints) or a textual ellipsis (Hahn et al., 1996) , not an issue in this paper.", "cite_spans": [ { "start": 312, "end": 330, "text": "Resolved(x, s, Us)", "ref_id": null }, { "start": 742, "end": 761, "text": "(Hahn et al., 1996)", "ref_id": "BIBREF5" } ], "ref_spans": [ { "start": 336, "end": 343, "text": "Table 3", "ref_id": "TABREF2" } ], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "The predicate lsReachable (cf. Table 4 ) requires ante to be reachable from the utterance Us associated with the segment level s. 2 Reachability is thus made dependent on the segment structure DS of the discourse as built up by the segmentation algorithm which is specified in Table 6 . In Table 4 , the symbol \"=str\" denotes string equality, N the natural numbers. We also introduce as a notational convention that a discourse segment is identified by its index s and its opening and closing utterance, viz. DS [s.beg] and DS [s.end] , respectively. Hence, we may either identify an utterance Ui by its linear text index, i, or, if it is accessible, with respect to its hierarchical discourse segment index, s (e.g., cf. Table 8 where", "cite_spans": [ { "start": 512, "end": 519, "text": "[s.beg]", "ref_id": null }, { "start": 527, "end": 534, "text": "[s.end]", "ref_id": null } ], "ref_spans": [ { "start": 31, "end": 38, "text": "Table 4", "ref_id": "TABREF3" }, { "start": 277, "end": 284, "text": "Table 6", "ref_id": "TABREF6" }, { "start": 290, "end": 297, "text": "Table 4", "ref_id": "TABREF3" }, { "start": 722, "end": 729, "text": "Table 8", "ref_id": "TABREF10" } ], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "U3 = UDs[1.end] or U13 = UDs[3.end]).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "The discourse segment index is always identical to the currently valid segment level, since the algorithm in Table 6 implements a stack behavior. Note also that we attach the discourse segment index s to center expressions, e.g., Cb(s, Us) . (Strobe & Hahn, 1996, p.272) and we here implicitly assume that they are accessed in the total order given.", "cite_spans": [ { "start": 230, "end": 239, "text": "Cb(s, Us)", "ref_id": null }, { "start": 242, "end": 270, "text": "(Strobe & Hahn, 1996, p.272)", "ref_id": null } ], "ref_spans": [ { "start": 109, "end": 116, "text": "Table 6", "ref_id": "TABREF6" } ], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "Resolved(x, s, Ui) := l ante if IsReachable(ante, s, Ui) A IsAnaphorFor(x, ante) under else", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "IsReachable(ante, s, Ui ) if ante 6 C/(s, Ui-1) else if ante E C/(s -1, Uosts_,.~,a]) else if (3v E N : ante =~tr Cp(v, UDsI .... a]) ^ v < (s -1)) A (-~Sv' 6 N: ante =,t,-Cp(v',UDst~,.~ndl) A v < v')", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": "ance Ui (selected by its linear text index, i). Lift only applies to structural configurations in the centering lists in which themes continuously shift at three different consecutive segment levels and associated preferred centers at least (cf. Table 2, lower box, for the basic pattern). Whenever a discourse segment is created, its starting and closing utterances are initialized to the current position in the discourse. Its end point gets continuously incremented as the analysis proceeds until this discourse segment DS is ultimately closed, i.e., whenever another segment DS' exists at the same or a hierarchically higher level of embedding such that the end point of DS' exceeds that of the end point of DS. Closed segments are inaccessible for the antecedent search. In Table 8 , e.g., the first two discourse segments at level 3 (ranging from U5 to U5 and Us to Ull ) are closed, while those at level 1 (ranging from U1 to U3), level 2 (ranging from U4 to UT) and level 3 (ranging from U12 to U13) are open.", "cite_spans": [], "ref_spans": [ { "start": 779, "end": 786, "text": "Table 8", "ref_id": "TABREF10" } ], "eq_spans": [], "section": "Computing Global Discourse Structure", "sec_num": "3" }, { "text": ":= Lift(s-1, i-1) if s>2Ai>3 ^ c.(s,u,_~) # c~(~ -1,u,_~) ^ c~(s -I, u,_~) # c.(s -2, u,_~) ^ c~(s,u,_,) \u2022 cj(s-1,u,_~) 8 else", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Lift(s, i)", "sec_num": null }, { "text": "The main algorithm (see Table 6 ) consists of three major logical blocks (s and Ui denote the current discourse segment level and utterance, respectively). ", "cite_spans": [], "ref_spans": [ { "start": 24, "end": 31, "text": "Table 6", "ref_id": "TABREF6" } ], "eq_spans": [], "section": "Lift(s, i)", "sec_num": null }, { "text": "The text with respect to which we demonstrate the working of the algorithm (see Table 7 ) is taken from a German computer magazine (c't, 1995, No.4, p.209) . For ease of presentation the text is somewhat shortened. Since the method for computing levels of discourse segments depends heavily on different kinds of anaphoric expressions, (pro)nominal anaphors and textual ellipses are marked by italics, and the (pro)nominal anaphors are underlined, in addition. In order to convey the influence of the German word order we provide a rough phrase-tophrase translation of the entire text.", "cite_spans": [ { "start": 131, "end": 155, "text": "(c't, 1995, No.4, p.209)", "ref_id": null } ], "ref_spans": [ { "start": 80, "end": 87, "text": "Table 7", "ref_id": null } ], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "The centered segmentation analysis of the sample text is given in Table 8 . The first column shows the linear text index of each utterance. The second column contains the centering data as computed by functional centering (Strube & Hahn, 1996) . The first element of the C I, the preferred center, Cp, is marked by bold font. The third column lists the centering transitions which are derived from the Cb/C! data of immediately successive utterances (cf. Table 1 for the definitions). The fourth column depicts the levels of discourse segments which are computed by the algorithm in Table 6 . Horizontal lines indicate the beginning of a segment (in the algorithm, this Table 6 ).", "cite_spans": [ { "start": 222, "end": 243, "text": "(Strube & Hahn, 1996)", "ref_id": "BIBREF5" } ], "ref_spans": [ { "start": 66, "end": 73, "text": "Table 8", "ref_id": "TABREF10" }, { "start": 455, "end": 462, "text": "Table 1", "ref_id": null }, { "start": 583, "end": 590, "text": "Table 6", "ref_id": "TABREF6" }, { "start": 670, "end": 677, "text": "Table 6", "ref_id": "TABREF6" } ], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "The computation starts at U1, the headline. The C1(Ux ) is set to \"1260\" which is meant as an abbreviation of \"Brother HL-1260\". Upon initialization, the beginning as well as the ending of the initial discourse segment are both set to \"1\". U2 and Ua simply continue this segment (block (1) of the algorithm), so Lift does not apply. The C v is set to \"1260\" in all utterances of this segment. Since U4 does neither contain any anaphoric expression which co-specifies the Cv(1 , Ua) (block (1)) nor any other element of the 67/( 1, U3) (block (2a)), and as there is no hierarchically preceding segment, block (2c) applies. The segment counter s is incremented and a new segment at level 2 is opened, setting the beginning and the ending to \"4\". The phrase \"das diinne Handbiichlein\" (the thin leaflet) in U5 does not co-specify the C v (2, U4) but co-specifies an element of the C! (2, U4) instead (viz. \"Handbuch\" (manual)).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "Hence, block (3) of the algorithm applies, leading to the creation of a new segment at level 3. The anaphor \"Handbuch\" (manual) in U6 co-specifies the Cv(3 , Us). Hence block (1) applies (the occurrence of \"1260\" in CI(U5 ) is due to the assumptions specified by Strube & ). Given this configuration, the function Lift lifts the embedded segment one level, so the", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "(3)", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "EQUATION", "cite_spans": [], "ref_spans": [], "eq_spans": [ { "start": 0, "end": 8, "text": "EQUATION", "ref_id": "EQREF", "raw_str": "(4)", "eq_num": "(5)" } ], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "Brother HL-1260", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "Ein Detail fiillt schon beim ersten Umgang mit dem grogen Brother auf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "One particular -is already noticed -in the first approach to -the big Brother.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "Im Betrieb macht e._gr durch ein kr~iftiges Arbeitsger~usch auf sich aufmerksam, das auch im Stand-by-Modus noch gut vemehmbar ist.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "In operation -draws -it -with a heavy noise levelattention to itself-which -also -in the stand-by modeis still well audible.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "F~r Standard-InstaUationen kommt man gut ohne Handbuch aus.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "As far as standard installations are concerned-gets -one well -by -without any manual. Table 7 : Sample Text segment which ended with U4 is now continued up to U6 at level 2. As a consequence, the centering data of U5 are excluded from further consideration as far as the co-specification by any subsequent anaphoric expression is concerned. Uz simply continues the same segment, since the textual ellipsis \"Seite\" (page) refers to \"Handbuch\" (manual). The utterances U8 to U10 exhibit a typical thematization-of-the-rhemes pattern which is quite common for the detailed description of objects. (Note the sequence of SHIFT transitions.) Hence, block (3) of the algorithm applies to each of the utterances and, correspondingly, new segments at the levels 3 to 5 are created. This behavior breaks down at the occurrence of the anaphoric expression \"sie\" (it) in Uxl which cospecifies the Cp ( 5, Ul o ), viz. \"auto-continue function\", denoted by another anaphoric expression, namely \"Hilfsfunktion\" (help function) in U10. Hence, block (1) applies. The evaluation of Lift succeeds with respect to two levels of embedding. As a result, the whole sequence is lifted up to level 3 and continues this segment which started at the discourse element \"lnhaltsverzeichhis\" (list of contents). As a result of applying Lift, the whole sequence is captured in one segment. U12 does not contain any anaphoric expression which co-specifies an element of the C! (3, U11), hence block (2) of the algorithm applies. The anaphor \"HL-1260\" does not cospecify the Cp of the utterance which represents the end of the hierarchically preceding discourse segment (UT), but it co-specifies an element of the C! (2, UT). The immediately preceding segment is ultimately closed and a parallel segment is opened at UI~ (cf. block (2b)). Note also that the algorithm does not check the C! (3, U10) despite the fact that it contains the antecedent of \"1260\". However, the occurrences of \"1260\" in the Cfs of U9", "cite_spans": [], "ref_spans": [ { "start": 87, "end": 94, "text": "Table 7", "ref_id": null } ], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "and Ux0 are mediated by textual ellipses. If these utterances contained the expression \"1260\" itself, the algorithm would have built a different discourse structure and, therefore, \"1260\" in U10 were reachable for the anaphor in Ulz. Segment 3, finally, is continued by Ulz.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "A Sample Text Segmentation", "sec_num": "4" }, { "text": "In this section, we present some empirical data concerning the centered segmentation algorithm. Our study was based on the analysis of twelve texts from the information technology domain (IT), of one text from a Ger-", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Empirical Evaluation", "sec_num": "5" }, { "text": "(1) Cb: Cf.\" (2) Cb: Cf: (3) Cb: Cf: (4) Cb:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Cf.\" (5) Cb: Cf: (6) Cb: Cf: (7)", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Cb: Cf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "(8) Cb: Cf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "(9) Cb: Cf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "(10) Cb: Cf: (11) Cb:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Cf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "(12) Cb: Cf: (13) Cb:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Cf:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Centering Data Trans.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "[ 1260, Graufl~ichen] man news magazine (Spiegel) 3, and of two literary texts 4 (Lit). Table 9 summarizes the total numbers of anaphors, textual ellipses, utterances, and words in the test set. In the test set, 139 anaphors (28%) and 116 textual ellipses (48,3%) fall out of the (intersentential) scope of Lit those common algorithms. So, the problem we consider is not a marginal one.", "cite_spans": [ { "start": 2, "end": 21, "text": "1260, Graufl~ichen]", "ref_id": null } ], "ref_spans": [ { "start": 88, "end": 95, "text": "Table 9", "ref_id": null } ], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Ui-2 Ui-a Ui-4 Ui-5 Table 9 : Test Set Table 10 and Table 11 consider the number of anaphoric and text-elliptical expressions, respectively, and the linear distance they have to their corresponding antecedents. Note that common centering algorithms (e.g., the one by Brennan et al. (1987) ) are specified only for the resolution of anaphors in Ui-1. They are 3japan -Der Neue der alten Garde. In Der Spiegel, Nr. 3, 1996. 4The first two chapters of a short story by the German writer Heiner MOiler (Liebesgeschichte. In Heiner MOiler. Geschichten aus der Produktion 2. Berlin: Rotbuch Verlag, 1974, pp.57-63) Ui-~ to Ui-lO 8 Ui-l, to Ui-15 3 Ui-l~ to U,-2o 1 Table 10 : Anaphoric Antecedent in Utterance U~ Table 12 and Table 13 give the success rate of the centered segmentation algorithm for anaphors and textual ellipses, respectively. The numbers in these tables indicate at which segment level anaphors and textual ellipses were correctly resolved. The category of errors", "cite_spans": [ { "start": 267, "end": 288, "text": "Brennan et al. (1987)", "ref_id": "BIBREF0" }, { "start": 409, "end": 421, "text": "Nr. 3, 1996.", "ref_id": null }, { "start": 569, "end": 608, "text": "Berlin: Rotbuch Verlag, 1974, pp.57-63)", "ref_id": null } ], "ref_spans": [ { "start": 20, "end": 27, "text": "Table 9", "ref_id": null }, { "start": 39, "end": 47, "text": "Table 10", "ref_id": null }, { "start": 52, "end": 60, "text": "Table 11", "ref_id": null }, { "start": 659, "end": 667, "text": "Table 10", "ref_id": null }, { "start": 707, "end": 728, "text": "Table 12 and Table 13", "ref_id": "TABREF1" } ], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "U/-1 Ui-2 Ui-3 Ui-4 Ui-5", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Ui-6 to Ui-lo IT Spiegel Lit E 94 15 15 124 42 6 8 56 16 0 0 16 14 0 0 14 8 0 0 8 14 1 0 15 7 0 0 7 Table 11 : Elliptical Antecedent in Utterance U covers erroneous analyses the algorithm produces, while the one for false positives concerns those resolution results where a referential expression was resolved with the hierarchically most recent antecedent but not with the linearly most recent (obviously, the targeted) one (both of them denote the same discourse entity). The categories Cy(s, Ui-1) in Tables 12 and 13 contain more elements than the categories Ui-1 in Tables 10 and 11 , respectively, due to the mediating property of textual ellipses in functional centering (Strube & Hahn, 1996) .", "cite_spans": [ { "start": 708, "end": 729, "text": "(Strube & Hahn, 1996)", "ref_id": "BIBREF5" } ], "ref_spans": [ { "start": 17, "end": 138, "text": "Spiegel Lit E 94 15 15 124 42 6 8 56 16 0 0 16 14 0 0 14 8 0 0 8 14 1 0 15 7 0 0 7 Table 11", "ref_id": "TABREF1" }, { "start": 601, "end": 617, "text": "Tables 10 and 11", "ref_id": null } ], "eq_spans": [], "section": "U~", "sec_num": null }, { "text": "Cp UDS[, , d] (2) (0) (5) The centered segmentation algorithm reveals a pretty good performance. This is to some extent implied by the structural patterns we find in expository texts, viz. their single-theme property (e.g., \"1260\" in the sample text). In contrast, the literary texts in the test exhibited a much more difficult internal structure which resembled the multiple thread structure of dialogues discussed by Ros6 et al. (1995) . The good news is that the segmentation procedure we propose is capable of dealing even with these more complicated structures. While only one antecedent of a pronoun was not reachable given the superimposed text structure, the remaining eight errors are characterized by full definite noun phrases or proper names. The vast majority of these phenomena can be considered informationally redundant utterances in the terminology of Walker (1996b) for which we currently have no solution at all. It seems to us that these kinds of phrases may override text-grammatical structures as evidenced by referential discourse segments and, rather, trigger other kinds of search strategies.", "cite_spans": [ { "start": 3, "end": 8, "text": "UDS[,", "ref_id": null }, { "start": 9, "end": 10, "text": ",", "ref_id": null }, { "start": 11, "end": 13, "text": "d]", "ref_id": null }, { "start": 419, "end": 437, "text": "Ros6 et al. (1995)", "ref_id": "BIBREF13" } ], "ref_spans": [], "eq_spans": [], "section": "U~ cI(~,U~-,)", "sec_num": null }, { "text": "Though we fed the centered segmentation algorithm with rather long texts (up to 84 utterances), the antecedents of only two anaphoric expressions had to bridge a hierarchical distance of more than 3 levels. This coincides with our supposition that the overall structure computed by the algorithm should be rather fiat. We could not find an embedding of more than seven levels.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "U~ cI(~,U~-,)", "sec_num": null }, { "text": "There has always been an implicit relationship between the local perspective of centering and the global view of focusing on discourse structure (cf. the discussion in Grosz et al. (1995) ). However, work establishing an explicit account of how both can be joined in a computational model has not been done so far. The efforts of Sidner (1983) , e.g., have provided a variety of different focus data structures to be used for reference resolution. This multiplicity and the on-going growth of the number of different entities (cf. Suri & McCoy (1994) ) mirrors an increase in explanatory constructs that we consider a methodological drawback to this approach because they can hardly be kept control of. Our model, due to its hierarchical nature implements a stack behavior that is also inherent to the above mentioned proposals. We refrain, however, from establishing a new data type (even worse, different types of stacks) that has to be managed on its own. There is no need for extra computations to determine the \"segment focus\", since that is implicitly given in the local centering data already available in our model.", "cite_spans": [ { "start": 168, "end": 187, "text": "Grosz et al. (1995)", "ref_id": "BIBREF3" }, { "start": 330, "end": 343, "text": "Sidner (1983)", "ref_id": "BIBREF15" }, { "start": 531, "end": 550, "text": "Suri & McCoy (1994)", "ref_id": "BIBREF18" } ], "ref_spans": [], "eq_spans": [], "section": "Related Work", "sec_num": "6" }, { "text": "A recent attempt at introducing global discourse notions into the centering framework considers the use of a cache model (Walker, 1996b) . This introduces an additional data type with its own management principles for data storage, retrieval and update. While our proposal for centered discourse segmentation also requires a data structure of its own, it is better integrated into centering than the caching model, since the cells of segment structures simply contain \"pointers\" that implement a direct link to the original centering data. Hence, we avoid extra operations related to feeding and updating the cache. The relation between our centered segmentation algorithm and Walker's (1996a) integration of centering into the cache model can be viewed from two different angles. On the one hand, centered segmentation may be a part of the cache model, since it provides an elaborate, nonlinear ordering of the elements within the cache. Note, however, that our model does not require any prefixed size corresponding to the limited attention constraint. On the other hand, centered segmentation may replace the cache model entirely, since both are competing models of the attentional state. Centered segmentation has also the additional advantage of restricting the search space of anaphoric antecedents to those discourse entities actually referred to in the discourse, while the cache model allows unrestricted retrieval in the main or long-term memory.", "cite_spans": [ { "start": 121, "end": 136, "text": "(Walker, 1996b)", "ref_id": "BIBREF20" }, { "start": 677, "end": 693, "text": "Walker's (1996a)", "ref_id": "BIBREF19" } ], "ref_spans": [], "eq_spans": [], "section": "Related Work", "sec_num": "6" }, { "text": "Text segmentation procedures (more with an information retrieval motivation, rather than being related to reference resolution tasks) have also been proposed for a coarse-grained partitioning of texts into contiguous, nonoverlapping blocks and assigning content labels to these blocks (Hearst, 1994) . The methodological basis of these studies are lexical cohesion indicators (Morris & Hirst, 1991) combined with word-level co-occurrence statistics. Since the labelling is one-dimensional, this approximates our use of preferred centers of discourse segments. These studies, however, lack the fine-grained information of the contents of Cf lists also needed for proper reference resolution.", "cite_spans": [ { "start": 285, "end": 299, "text": "(Hearst, 1994)", "ref_id": "BIBREF7" }, { "start": 376, "end": 398, "text": "(Morris & Hirst, 1991)", "ref_id": "BIBREF9" } ], "ref_spans": [], "eq_spans": [], "section": "Related Work", "sec_num": "6" }, { "text": "Finally, many studies on discourse segmentation highlight the role of cue words for signaling segment boundaries (cf., e.g., the discussion in Passonneau & Litman (1993) ). However useful this strategy might be, we see the danger that such a surface-level description may actually hide structural regularities at deeper levels of investigation illustrated by access mechanisms for centering data at different levels of discourse segmentation.", "cite_spans": [ { "start": 143, "end": 169, "text": "Passonneau & Litman (1993)", "ref_id": "BIBREF11" } ], "ref_spans": [], "eq_spans": [], "section": "Related Work", "sec_num": "6" }, { "text": "We have developed a proposal for extending the centering model to incorporate the global referential structure of discourse for reference resolution. The hierarchy of discourse segments we compute realizes certain constraints on the reachability of antecedents. Moreover, the claim is made that the hierarchy of discourse segments implements an intuitive notion of the limited attention constraint, as we avoid a simplistic, cognitively implausible linear backward search for potentional discourse referents. Since we operate within a functional framework, this study also presents one of the rare formal accounts of thematic progression patterns for full-fledged texts which were informally introduced by Dane~ (1974) .", "cite_spans": [ { "start": 706, "end": 718, "text": "Dane~ (1974)", "ref_id": "BIBREF2" } ], "ref_spans": [], "eq_spans": [], "section": "Conclusions", "sec_num": "7" }, { "text": "The model, nevertheless, still has several restrictions. First, it has been developed on the basis of a small corpus of written texts. Though these cover diverse text sorts (viz. technical product reviews, newspaper articles and literary narratives), we currently do not account for spoken monologues as modelled, e.g., by Passonneau & Litman (1993) or even the intricacies of dyadic conversations Ros6 et al. (1995) deal with. Second, a thorough integration of the referential and intentional description of discourse segments still has to be worked out.", "cite_spans": [ { "start": 323, "end": 349, "text": "Passonneau & Litman (1993)", "ref_id": "BIBREF11" }, { "start": 398, "end": 416, "text": "Ros6 et al. (1995)", "ref_id": "BIBREF13" } ], "ref_spans": [], "eq_spans": [], "section": "Conclusions", "sec_num": "7" } ], "back_matter": [ { "text": "Acknowledgments. We like to thank our colleagues in the CLIF group for fruitful discussions and instant support, Joe Bush who polished the text as a native speaker, the three anonymous reviewers for their critical comments, and, in particular, Bonnie Webber for supplying invaluable comments to an earlier draft of this paper. Michael Strube is supported by a postdoctoral grant from DFG (Str 545/1-1).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "acknowledgement", "sec_num": null } ], "bib_entries": { "BIBREF0": { "ref_id": "b0", "title": "A centering approach to pronouns", "authors": [ { "first": "S", "middle": [ "E" ], "last": "Brennan", "suffix": "" }, { "first": "M", "middle": [ "W J" ], "last": "Friedman & C", "suffix": "" }, { "first": "", "middle": [], "last": "Pollard", "suffix": "" } ], "year": 1987, "venue": "Proc. of the 25 th Annual Meeting of the Association for Computational Linguistics", "volume": "", "issue": "", "pages": "", "other_ids": {}, "num": null, "urls": [], "raw_text": "Brennan, S. E., M. W. Friedman & C. J. Pollard (1987). A centering approach to pronouns. 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Computational Linguistics, 22(2):255-264.", "links": null } }, "ref_entries": { "FIGREF0": { "type_str": "figure", "uris": null, "text": "(s', i').end] := i else if~3r E Tt : r \u2022 Cl(s, Ui_l ) (2a) then found := FALSE k:~s while-,found A (k > 1) k:=k-1 i_f3r \u2022 7?.: r =s,r Cp(k, Utk.~,,~) then s := k DS[s.end] := i found := TRUE else if k = s -1 (2b) then if3r \u2022~:r\u2022 Cs(k, Utk.o,,,~) then DS[s.beg] := i DS[s.end] .beg] := i DS[s.end] := i", "num": null }, "FIGREF1": { "type_str": "figure", "uris": null, "text": "corresponds to a value assignment to DS[s.beg]). Vertical lines show the extension of a segment (its end is fixed by an assignment to DS[s.end]). The fifth column indicates which block of the algorithm applies to the current utterance (cf. the right margin in", "num": null }, "TABREF1": { "num": null, "type_str": "table", "text": "Thematic Progression Patterns", "html": null, "content": "" }, "TABREF2": { "num": null, "type_str": "table", "text": "Resolution of Anaphora", "html": null, "content": "
" }, "TABREF3": { "num": null, "type_str": "table", "text": "Reachability of the Anaphoric AntecedentFinally, the function Lift(s, i) (cf.Table 5) determines the appropriate discourse segment level, s, of an utter-2The Cf lists in the functional centering model are totally", "html": null, "content": "
ordered
" }, "TABREF4": { "num": null, "type_str": "table", "text": "Lifting to the Appropriate Discourse Segment", "html": null, "content": "" }, "TABREF5": { "num": null, "type_str": "table", "text": "Segment. The Cp(s, Ui-1) is taken over for Ui. If Ui-1 and Ui indicate the end of a sequence in which a series of thematizations of rhemes have occurred, all embedded segments are lifted by the function Lift to a higher level s'. As a result of lifting, the entire sequence (including the final two utterances) forms a single segment. This is trivially true for cases of a constant theme. If none of the anaphoric expressions under consideration co-specify the Cp(8 -1, U[8_l.end]), then the entire C! at this segment level is checked for the given utterance. If an antecedent matches, the segment which contains Ui-1 is ultimately closed, since Ui opens a parallel segment at the same level of embedding. Subsequent anaphora checks exclude any of the preceding parallel segments from the search for a valid antecedent and just visit the currently open one. If none of the above cases applies, then for utterance Ui a new, embedded segment is opened. In the course of processing the following utterances, this decision may be retracted by the function Lift. It serves as a kind", "html": null, "content": "
2. Close Embedded Segment(s). (a) Close the embedded segment(s) and continue another, already existing segment: If Ui does not include any anaphoric expression which is an element of the Cf (s, Ui-O, then match the antecedent in the hierarchically reachable seg-ments. Only the Cp of the utterance at the end point of any of these segments is considered a potential antecedent. Note that, as a side effect, hierarchically lower segments are ulti-mately closed when a match at higher segment levels succeeds. (b) Close the embedded segment and open a new, parallel one: (c) Open new, embedded segment: If there is no matching antecedent in hierarchically reach-able segments, then for utterance Ui a new, em-bedded segment is opened. i:=i+1 n := {Resolved(x,s, Ui) lx E U~} if3r \u2022 T~ : r ~---str Cp(s, Ui-1)
" }, "TABREF6": { "num": null, "type_str": "table", "text": "", "html": null, "content": "" }, "TABREF7": { "num": null, "type_str": "table", "text": "Kein Wander: unter dem lnhaltsverzeichnis steht der lapidare Hinweis, man m6ge sich die Seiten dieses Kapitels doch bitte yon Diskette ausdrucken-Frechheit.No wonder: beneath the table of contentsone finds the terse instruction, one should -oneself-the pages of this section -please -from disk -print out --impertinence.", "html": null, "content": "
(8) (9) Ohne diesen Ausdruck sucht man vergebens nach einem
Hinweis darauf, warum die Auto-Continue-Funktion in
der PostScript-Emulation nicht wirkt.
Without this print-out, looks -one -in vain -for a hint -
why -the auto-continue-function -in the PostScript em-
ulation -does not work.
(10) Nach dem Einschalten zeigt das LC-Display an, dab diese
praktische Hilfsfunktion nicht aktiv ist;
After switching on -depicts -the LC display -that -this
practical help function -not active -is;
Zwar ed~iutert das dSnne Handbiichlein die Bedienung
der Hardware anschaulich und gut illustriert.(11) si__. ge tiberwacht den Dateientransfer vom Computer.
Admittedly, gives -the thin leaflet-the operation of theit monitors the file transfer from the computer.
hardware-a clear description of -and -well illustrated.
(12) Viele der kleinen Macken verzeiht man dem HL-1260
Die Software-Seite wurde im Handbuch dagegenwenn man erste Ausdrucke in H~inden h~ilt.
stiefmSttedich behandelt:Many of the minor defects -pardons -one -the
The software part -was -in the manual-however -likeHL-1260, when -one -the first print outs -holds in
a stepmother-treated:[one' s] hands.
for further
information.
" }, "TABREF10": { "num": null, "type_str": "table", "text": "Sample of a Centered Text Segmentation Analysis neither specified for anaphoric antecedents in Ui, not an issue here, nor for anaphoric antecedents beyond Ui-1.", "html": null, "content": "" }, "TABREF13": { "num": null, "type_str": "table", "text": "", "html": null, "content": "
: Anaphoric Antecedent in Center~
ITSpiegel Lit
cl (s, U~-i )1561817
Cp(s -1, UDSi,-1.,,~d])1804
CI(s -1, Uosls-~.*,a])1012
Cp(s -2, Uosts-~.~,,~l)710
Cp(s -3, UDats-Z.ena])300
errors120
" }, "TABREF15": { "num": null, "type_str": "table", "text": "Elliptical Antecedent in Centerx", "html": null, "content": "" } } } }