{ "paper_id": "P92-1030", "header": { "generated_with": "S2ORC 1.0.0", "date_generated": "2023-01-19T08:12:08.957260Z" }, "title": "TENSE TREES AS THE \"FINE STRUCTURE\" OF DISCOURSE", "authors": [ { "first": "Chung", "middle": [ "Hee" ], "last": "Hwang", "suffix": "", "affiliation": { "laboratory": "", "institution": "University of Rochester Rochester", "location": { "postCode": "14627", "region": "New York", "country": "U. S. A" } }, "email": "hwang@edu" }, { "first": "Lenhart", "middle": [ "K" ], "last": "Schubert", "suffix": "", "affiliation": { "laboratory": "", "institution": "University of Rochester Rochester", "location": { "postCode": "14627", "region": "New York", "country": "U. S. A" } }, "email": "schubert@cs.rochester@edu" } ], "year": "", "venue": null, "identifiers": {}, "abstract": "We present a new compositional tense-aspect deindexing mechanism that makes use of tense trees as components of discourse contexts. The mechanism allows reference episodes to be correctly identified even for embedded clauses and for discourse that involves shifts in temporal perspective, and permits deindexed logical forms to be automatically computed with a small number of deindexing rules.", "pdf_parse": { "paper_id": "P92-1030", "_pdf_hash": "", "abstract": [ { "text": "We present a new compositional tense-aspect deindexing mechanism that makes use of tense trees as components of discourse contexts. The mechanism allows reference episodes to be correctly identified even for embedded clauses and for discourse that involves shifts in temporal perspective, and permits deindexed logical forms to be automatically computed with a small number of deindexing rules.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Abstract", "sec_num": null } ], "body_text": [ { "text": "Work on discourse structure, e.g., [Reichman, 1985; Grosz and Sidner, 1986; Allen, 1987] , has so far taken a rather coarse, high-level view of discourse, mostly treating sentences or sentence-like entities (\"utterance units, .... contributions,\" etc.) as the lowest-level discourse elements. To the extent that sentences are analyzed at all, they are simply viewed as carriers of certain features relevant to supra-sentential discourse structure: cue words, tense, time adverbials, aspectual class, intonational cues, and others. These features are presumed to be extractable in some straightforward fashion and provide the inputs to a higher-level discourse segment analyzer.", "cite_spans": [ { "start": 35, "end": 51, "text": "[Reichman, 1985;", "ref_id": "BIBREF12" }, { "start": 52, "end": 75, "text": "Grosz and Sidner, 1986;", "ref_id": "BIBREF2" }, { "start": 76, "end": 88, "text": "Allen, 1987]", "ref_id": "BIBREF0" }, { "start": 207, "end": 252, "text": "(\"utterance units, .... contributions,\" etc.)", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "However, sentences (or their logical forms) are not in general \"flat,\" with a single level of structure and features, but may contain multiple levels of clausal and adverbial embedding. This substructure can give rise to arbitrarily complex relations among the contributions made by the parts, such as temporal and discourse relations among subordinate clausal constituents and events or states of affairs they evoke. It is therefore essential, in a comprehensive analysis of discourse structure, that these intra-sentential relations be systematically brought to light and integrated with larger-scale discourse structures.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "Our particular interest is in tense, aspect and other indicators of temporal structure. We are developing a uniform, compositional approach to interpretation in which a parse tree leads directly (in rule-to-rule fash-ion) to a preliminary, indezical logical form, and this LF is deindezed by processing it in the current context (a well-defined structure). Deindexing simultaneously transforms the LF and the context: context-dependent constituents of the LF, such as operators past, pres and perf and adverbs like today or earlier, are replaced by explicit relations among quantified episodes; (anaphora are also deindexed, but this is not discussed here); and new structural components and episode tokens (and other information) are added to the context. This dual transformation is accomplished by simple recursive equivalences and equalities. The relevant context structures are called tense trees; these are what we propose as the \"fine structure\" of discourse, or at least as a key component of that fine structure.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "In this paper, we first review Reichenbach's influential work on tense and aspect. Then we describe temporal deindexing using tense trees, and extensions of the mechanism to handle discourse involving shifts in temporal perspective.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Introduction", "sec_num": "1" }, { "text": "Researchers concerned with higher-level discourse structure, e.g., Webber [1987; 1988] , Passonneau [1988] and Song and Cohen [1991] , have almost invariably relied on some Reichenbach [1947] -1ike conception of tense. The syntactic part of this conception is that there are nine tenses in English, namely simple past, present and future tense, past, present and future perfect tense, and posterior past, present and future tense 1 (plus progressive variants). The semantic part of the conception is that each tense specifies temporal relations among exactly three times particular to a tensed clause, namely the event time (E), the reference time (R) and the speech time (S). On this conception, information in discourse is a matter of \"extracting\" one of the nine Reichenbachian tenses from each sentence, asserting the 1Exarnples of expressions in posterior tense are would, was going to (posterior past), is going to (posterior present), and will be going to (posterior future). appropriate relations among E, R and S, and appropriately relating these times to previously introduced times, taking account of discourse structure cues implicit in tense shifts.", "cite_spans": [ { "start": 67, "end": 80, "text": "Webber [1987;", "ref_id": "BIBREF16" }, { "start": 81, "end": 86, "text": "1988]", "ref_id": "BIBREF17" }, { "start": 100, "end": 106, "text": "[1988]", "ref_id": null }, { "start": 111, "end": 132, "text": "Song and Cohen [1991]", "ref_id": "BIBREF15" }, { "start": 173, "end": 191, "text": "Reichenbach [1947]", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "Farewell to Reichenbach", "sec_num": "2" }, { "text": "It is easy to understand the appeal of this approach when one's concern is with higher-level structure. By viewing sentences as essentially flat, carrying tense as a top-level feature with nine possible values and evoking a triplet of related times, one can get on with the higherlevel processing with minimum fuss. But while there is much that is right and insightful about Reichenbach's conception, it seems to us unsatisfactory from a modern perspective. One basic problem concerns embedded clauses. Consider, for instance, the following passage.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Farewell to Reichenbach", "sec_num": "2" }, { "text": "(1) John will find this note when he gets home.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Farewell to Reichenbach", "sec_num": "2" }, { "text": "(2) He will think(a) Mary has left(b).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Farewell to Reichenbach", "sec_num": "2" }, { "text": "Reichenbach's analysis of (2) gives us Eb < S, Rb < Ra, Ea, where tl < t~ means tl is before t2, as below.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Farewell to Reichenbach", "sec_num": "2" }, { "text": "That is, John will think that Mary's leaving took place some time before the speaker uttered sentence (2). This is incorrect; it is not even likely that John would know about the utterance of (2). In actuality, (2) only implies that John will think Mary's leaving took place some time before the time of his thinking, i.e., S < Ra, Ea and Eb < Rb, Ra , as shown below.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Rb R~ E~", "sec_num": null }, { "text": "Ra,E~", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "S ~", "sec_num": null }, { "text": "Thus, Reichenbach's system fails to take into account the local context created by syntactic embedding. Attempts have been made to refine Reichenbach's theory (e.g., [Hornstein, 1977; Smith, 1978; Nerbonne, 1986] ), but we think the lumping together of tense and aspect, and the assignment of E, R, S triples to all clauses, are out of step with modern syntax and semantics, providing a poor basis for a systematic, compositional account of temporal relations within clauses and between clauses. In particular, we contend that English past, present, future and perfect are separate morphemes making separate contributions to syntactic structure and meaning. Note that perfect have, like most verbs, can occur untensed (\"She is likely to have left by now\"). Therefore, if the meaning of other tensed verbs such as walks or became is regarded as composite, with the tense morpheme supplying a \"present\" or \"past\" component of the meaning, the same ought to be said about tensed forms of have. The modals will and would do not have untensed forms. Nevertheless, considerations of syntactic and semantic uniformity suggest that they too have composite meanings, present or past tense being one part and \"future modality\" the other. This unifies the analyses of the modals in sentences like \"He knows he will see her again\" and \"He knew he would see her again,\" and makes them entirely parallel to paraphrases in terms of going to, viz., \"He knows he is going to see her again\" and \"He knew he was going to see her again.\" We take these latter \"posterior tense\" forms to be patently hierarchical (e.g., is going to see her has 4 levels of VP structure, counting to as an auxiliary verb) and hence semantically composite on any compositional account. Moreover, going to can both subordinate, and be subordinated by, perfect have, as in \"He is going to have left by then.\" This leads to additional \"complex tenses\" missing from Reichenbach's list.", "cite_spans": [ { "start": 166, "end": 183, "text": "[Hornstein, 1977;", "ref_id": "BIBREF5" }, { "start": 184, "end": 196, "text": "Smith, 1978;", "ref_id": null }, { "start": 197, "end": 212, "text": "Nerbonne, 1986]", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "Eb f Rb", "sec_num": null }, { "text": "We therefore offer a compositional account in which operators corresponding to past (past), present (pres), future (futr) and perfect (perf) contribute separately and uniformly to the meanings of their operands, i.e., formulas at the level of LF. Thus, for instance, the temporal relations implicit in \"John will have left\" are ob-tMned not by extracting a \"future perfect\" and asserting relations among E, R and S, but rather by successively taking account of the meanings of the nested pres, futr and perf operators in the LF of the sentence. As it happens, each of those operators implicitly introduces exactly one episode, yielding a Reichenbach-like result in this case. (But note: a simple present sentence like \"John is tired\" would introduce only one episode concurrent with the speech time, not two, as in Reichenbach's analysis.) Even more importantly for present purposes, each ofpres, past, futr and perf is treated uniformly in deindexing and context change. More specifically, they drive the generation and traversal of tense trees in deindexing.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Eb f Rb", "sec_num": null }, { "text": "Tense trees provide that part of a discourse context structure 2 which is needed to interpret (and deindex) temporal operators and modifiers within the logical form of English sentences. They differ from simple lists of Reichenbachian indices in that they organize episode tokens (for described episodes and the utterances themselves) in a way that echoes the hierarchy of temporal and modal operators of the sentences and clauses from which the tokens arose. In this respect, they are anal-ogous to larger-scale representations of discourse structure which encode the hierarchic segment structure of discourse. (As will be seen, the analogy goes further.) Tense trees for successive sentences are \"overlaid\" in such a way that related episode tokens typically end up as adjacent elements of lists at tree nodes. The traversal of trees and the addition of new tokens is simply and fully determined by the logical forms of the sentences being interpreted. The major advantage of tense trees is that they allow simple, systematic interpretation (by deindexing) of tense, aspect, and time adverbials in texts consisting of arbitrarily complex sentences, and involving implicit temporal reference across clause and sentence boundaries. This includes certain relations implicit in the ordering of clauses and sentences. As has been frequently observed, for a sequence of sentences within the same discourse segment, the temporal reference of a sentence is almost invariably connected to that of the previous sentence in some fashion. Typically, the relation is one of temporal precedence or concurrency, depending on the aspectual class or aktionsart involved (eft, \"John closed his suitcase; He walked to the door\" versus \"John opened the door; Mary was sleeping\"). However, in \"Mary got in her Ferrari. She bought it with her own money,\" the usual temporal precedence is reversed (based on world knowledge). Also, other discourse relations could be implied, such as cause-of, explains, elaborates, etc. (more on this later). Whatever the relation may be, finding the right pair of episodes involved in such relations is of crucial importance for discourse understanding. Echoing Leech [1987, p41] , we use the predicate constant orients, which subsumes all such relations. Note that the orients predications can later be used to make probabilistic or default inferences about the temporal or causal relations between the two episodes, based on their aspectual class and other information. In this way they supplement the information provided by larger-scale discourse segment structures. We now describe tense trees more precisely.", "cite_spans": [ { "start": 2182, "end": 2193, "text": "[1987, p41]", "ref_id": null } ], "ref_spans": [], "eq_spans": [], "section": "Tense Trees", "sec_num": "3" }, { "text": "The form of a tense tree is illustrated in Figure 1 . As an aid to intuition, the nodes in Figure 1 are annotated with simple sentences whose indexical LFs would lead to those nodes in the course of deindexing. A tense tree node may have up to three branches--a leftward past branch, a downward perfect branch, and a rightward future branch. Each node contains a stack-like list of recently introduced episode tokens (which we will often refer to simply as episodes).", "cite_spans": [], "ref_spans": [ { "start": 43, "end": 51, "text": "Figure 1", "ref_id": null }, { "start": 91, "end": 99, "text": "Figure 1", "ref_id": null } ], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "In addition to the three branches, the tree may have (horizontal) embedding links to the roots of embedded tense trees. There are two kinds of these embedding links, both illustrated in of the utterance context. The two kinds of embedding links require slightly different tree traversal techniques as will be seen later. A set of trees connected by embedding links is called a tense tree structure (though we often refer loosely to tense tree structures as tense trees). This is in effect a tree of tense trees, since a tense tree can be embedded by only one other tree. At any time, exactly one node of the tense tree structure for a discourse is in focus, and the focal node is indicated by ~). Note that the \"tense tree\" in Figure 1 is in fact a tense tree structure, with the lowest node in focus.", "cite_spans": [], "ref_spans": [ { "start": 727, "end": 735, "text": "Figure 1", "ref_id": null } ], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "By default, an episode added to the right end of a list at a node is \"oriented\" by the episode which was previously rightmost. For episodes stored at different nodes, we can read off their temporal relations from the tree roughly as follows. At any given moment, for a pair of episodes e and e' that are rightmost at nodes n and n', respectively, where n' is a daughter of n, if the branch connecting the two nodes is a past branch, Is' before e]3; if it is a perfect branch, [e' impinges-on e] (as we explain later, this yields entailments [e' before e] if e' is nonstative and [e' until e] if e' is stative, respectively illustrated by \"John has left\" and \"John has been working\"); if it is a future branch, [d after e]; and if it is an embedding link, [d at-about e]. These orienting relations and temporal relations are not extracted post hoc, but rather are automatically asserted in the course of deindexing using the rules shown later.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "As a preliminary example, consider the following passage and a tense tree annotated with episodes derived from it by our deindexing rules:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "(3) John picked up the phone. 4 and such a sentence describes a state in which & has occurred--in this instance, a state in which John has told Mary that he will call her. It is this stativity of el which (by default) leads to a same-time interpretation of orients. 4 Thus, on our account, the tendency of past perfect \"reference time\" to align itself with a 3Or, sometimes, same-time (cf., \"John noticed that Mary looked pale\" vs. \"Mary realized that someone broke her vase\"). This is not decided in an ad hoc manner, but as a result of systematically interpreting the context-charged relation belT. previously introduced past event is just an instance of a general tendency of stative episodes to align themselves with their orienting episode. This is the same tendency noted previously for \"John opened the door. Mary was sleeping.\" We leave further comments about particularizing the orients relation to a later subsection. We remarked that the relation [e2 same-time etett] is obtained directly from the deindexing rules. We leave it to the reader to verify this in detail (see Past and Futr rules stated below). We note only that e2 is evoked by the past tense component of would in 4, and denotes a (possible) state in which John will call Mary.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "Its stativity, and the fact that the subordinate clause in (4) is \"past-dominated, ''5 causes [e2 bef T eteu] to be deindexed to [e2 same-time etch].", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "We now show how tense trees are modified as discourse is processed, in particular, how episode tokens are stored at appropriate nodes of the tense tree, and how deindexed LFs, with orients and temporal ordering relations incorporated into them, are obtained.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Tense Tree Structure", "sec_num": null }, { "text": "The processing of the (indexical) LF of a new utterance always begins with the root node of the current tense tree (structure) in focus. The processing of the top-level operator immediately pushes a token for the surface speech act onto the episode list of the root node.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": "Here is a typical indexical LF:", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": "( decl (past [John know (That (past (', (perf [Mary leave]))))])", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": ") \"John knew that Mary had not left.\"", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": "(decl stands for declarative; its deindexing rule introduces the surface speech act of type \"tell\"). As mentioned earlier, our deindexing mechanism is a compositional one in which operators past, futr, perf, -,, That, decl, etc., contribute separately to the meaning of their operands. As the LF is recursively transformed, the tense and aspect operators encountered, past, perf and futr, in particular, cause the focus to shift \"downward\" along existing branches (or new ones if necessary). That is, processing a past operator shifts the current focus down to the left, creating a new branch if necessary. The resulting tense tree is symbolized as /T. Similarly perf shifts straight down, and futr shifts down to the right, with respective results t T and \\ T. pres maintains the current focus. Certain operators embed new trees at the current node, written ~--~T (e.g., That), or shift focus to an existing embedded tree, written \u00a2--*T (e.g., decl). Focus shifts to a parent or embedding node are symbolized as T T and .--T respectively. As a final tree operation, OT denotes storage of episode token e T (a new episode symbol not yet used in T) at the current 5A node is past-domlnated if there is a past branch in its ancestry (where embedding finks also count as ancestry links).", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": "focus, as rightmost element of its episode list. As each node comes into focus, its episode list and the lists at certain nodes on the same tree path provide explicit reference episodes in terms of which past, pres, futr, pert, time adverbials, and implicit \"orienting\" relations are rewritten nonindexically. Eventually the focus returns to the root, and at this point, we have a nonindexical LF, as well as a modified tense tree.", "cite_spans": [], "ref_spans": [], "eq_spans": [], "section": "Processin~ of Utterances", "sec_num": null }, { "text": "Before we proceed with an example, we show some of the basic deindexing rules here. 6 In the following,\"**\" is an episodic operator that connects a formula with the situation it characterizes. Predicates are infixed and quantifiers have restrictions (following a colon), r Tree transform: (That