File size: 54,262 Bytes
6fa4bc9 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 | {
"paper_id": "P99-1011",
"header": {
"generated_with": "S2ORC 1.0.0",
"date_generated": "2023-01-19T09:31:27.863556Z"
},
"title": "A Meta-Level Grammar: Redefining Synchronous TAG for Translation and Paraphrase",
"authors": [
{
"first": "Mark",
"middle": [],
"last": "Dras",
"suffix": "",
"affiliation": {
"laboratory": "",
"institution": "Macquarie University",
"location": {
"country": "Australia"
}
},
"email": "markd@\u00b1cs@mq.edu.au"
}
],
"year": "",
"venue": null,
"identifiers": {},
"abstract": "In applications such as translation and paraphrase, operations are carried out on grammars at the meta level. This paper shows how a meta-grammar, defining structure at the meta level, is useful in the case of such operations; in particular, how it solves problems in the current definition of Synchronous TAG (Shieber, 1994) caused by ignoring such structure in mapping between grammars, for applications such as translation. Moreover, essential properties of the formalism remain unchanged.",
"pdf_parse": {
"paper_id": "P99-1011",
"_pdf_hash": "",
"abstract": [
{
"text": "In applications such as translation and paraphrase, operations are carried out on grammars at the meta level. This paper shows how a meta-grammar, defining structure at the meta level, is useful in the case of such operations; in particular, how it solves problems in the current definition of Synchronous TAG (Shieber, 1994) caused by ignoring such structure in mapping between grammars, for applications such as translation. Moreover, essential properties of the formalism remain unchanged.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "Abstract",
"sec_num": null
}
],
"body_text": [
{
"text": "A grammar is, among other things, a device by which it is possible to express structure in a set of entities; a grammar formalism, the constraints on how a grammar is allowed to express this. Once a grammar has been used to express structural relationships, in many applications there are operations which act at a 'meta level' on the structures expressed by the grammar: for example, lifting rules on a dependency grammar to achieve pseudo-projectivity (Kahane et al, 1998) , and mapping between synchronised Tree Adjoining Grammars (TAGs) (Shieber and Schabes, 1990; Shieber 1994 ) as in machine translation or syntax-to-semantics transfer. At this meta level, however, the operations do not themselves exploit any structure. This paper explores how, in the TAG case, using a meta-level grammar to define meta-level structure resolves the flaws in the ability of Synchronous TAG (S-TAG) to be a representation for applications such as machine translation or paraphrase.",
"cite_spans": [
{
"start": 454,
"end": 474,
"text": "(Kahane et al, 1998)",
"ref_id": null
},
{
"start": 541,
"end": 568,
"text": "(Shieber and Schabes, 1990;",
"ref_id": "BIBREF8"
},
{
"start": 569,
"end": 581,
"text": "Shieber 1994",
"ref_id": "BIBREF9"
}
],
"ref_spans": [],
"eq_spans": [],
"section": "Introduction",
"sec_num": "1"
},
{
"text": "This paper is set out as follows. It describes the expressivity problems of S-TAG as noted in Shieber (1994) , and shows how these occur also in syntactic paraphrasing. It then demonstrates, illustrated by the relative structural complexity which occurs at the meta level in syntactic paraphrase, how a meta-level grammar resolves the representational problems; and it further shows that this has no effect on the generative capacity of S-TAG.",
"cite_spans": [
{
"start": 94,
"end": 108,
"text": "Shieber (1994)",
"ref_id": "BIBREF9"
}
],
"ref_spans": [],
"eq_spans": [],
"section": "Introduction",
"sec_num": "1"
},
{
"text": "Synchronous TAG, the mapping between two Tree Adjoining Grammars, was first proposed by Shieber and Schabes (1990) . An application proposed concurrently with the definition of S-TAG was that of machine translation, mapping between English and French (Abeill~ et al, 1990) ; work continues in the area, for example using S-TAG for English-Korean machine translation in a practical system (Palmer et al, 1998) . In mapping between, say, English and French, there is a lexicalised TAG for each language (see XTAG, 1995, for an overview of such a grammar). Under the definition of TAG, a grammar contains elementary trees, rather than flat rules, which combine together via the operations of substitution and adjunction (composition operations) to form composite structures--derived trees--which will ultimately provide structural representations for an input string if this string is grammatical. An overview of TAGs is given in Joshi and Schabes (1996) .",
"cite_spans": [
{
"start": 88,
"end": 114,
"text": "Shieber and Schabes (1990)",
"ref_id": "BIBREF8"
},
{
"start": 232,
"end": 272,
"text": "English and French (Abeill~ et al, 1990)",
"ref_id": null
},
{
"start": 388,
"end": 408,
"text": "(Palmer et al, 1998)",
"ref_id": "BIBREF5"
},
{
"start": 927,
"end": 951,
"text": "Joshi and Schabes (1996)",
"ref_id": "BIBREF4"
}
],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "The characteristics of TAGs make them better suited to describing natural language than Context Free Grammars (CFGs): CFGs are not adequate to describe the entire syntax of natural language (Shieber, 1985) , while TAGs are able to provide structures for the constructions problematic for CFGs, and without a much greater generative capacity. Two particular chaxacteris- tics of TAG that make it well suited to describing natural language are the extended domain of locality (EDL) and factoring recursion from the domain of dependencies (FRD). In TAG, for instance, information concerning dependencies is given in one tree (EDL): for example, in Figure 1,1 the information that the verb defeated has subject and object arguments is contained in the tree al. In a CFG, with rules of the form S --+ NP VP and VP --+ V NP, it is not possible to have information about both arguments in the same rule unless the VP node is lost. TAG keeps dependencies together, or local, no matter how far apart the corresponding lexicM items are. FRD means that recursive information--for example, a sequence of adjectives modifying the object noun of defeated--are factored out into separate trees, leaving dependencies together.",
"cite_spans": [
{
"start": 190,
"end": 205,
"text": "(Shieber, 1985)",
"ref_id": "BIBREF7"
}
],
"ref_spans": [
{
"start": 645,
"end": 651,
"text": "Figure",
"ref_id": null
}
],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "A consequence of the TAG definition is that, unlike CFG, a TAG derived tree is not a record of its own derivation. In CFG, each tree given as a structural description to a string enables the rules applied to be recovered. In a TAG, this is not possible, so each derived tree has an associated derivation tree. If the trees in Figure 1 were composed to give a structural description for Garrad cunningly defeated the Sumerians, tion tree would be as in Figure 2 . 2 Weir (1988) terms the derived tree, and its component elementary trees, OBJECT-LEVEL TREES; the derivation tree is termed a META-LEVEL TREE, since it describes the object-level trees. The derivation trees are context free (Weir, 1988) , that is, they can be expressed by a CFG; Weir showed that applying a TAG yield function to a context free derivation tree (that is, reading the labels off the tree, and substituting or adjoining the corresponding objectlevel trees as appropriate) will uniquely specify a TAG tree. Schabes and Shieber (1994) characterise this as a function 7) from derivation trees to derived trees.",
"cite_spans": [
{
"start": 465,
"end": 476,
"text": "Weir (1988)",
"ref_id": "BIBREF10"
},
{
"start": 687,
"end": 699,
"text": "(Weir, 1988)",
"ref_id": "BIBREF10"
},
{
"start": 983,
"end": 1009,
"text": "Schabes and Shieber (1994)",
"ref_id": "BIBREF6"
}
],
"ref_spans": [
{
"start": 326,
"end": 334,
"text": "Figure 1",
"ref_id": null
},
{
"start": 452,
"end": 460,
"text": "Figure 2",
"ref_id": "FIGREF1"
}
],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "The idea behind S-TAG is to take two TAGs and link them in an appropriate way so that when substitution or adjunction occurs in a tree in one grammar, then a corresponding composition operation occurs in a tree in the other grammar. In these tree pairs, a diacritic ([-/7) represents a link between the trees, such that if a substitution or adjunction occurs at one end of the link, a corresponding operation must occur at the other end, which is situated in the other tree of the same tree pair. Thus if the tree for John in a7 is substituted at E] in the left tree of a6, the tree for Jean must be substituted at [-~ in the right tree. The diacritic E] allows a sentential modifier for both trees (e.g. unfortunately / malheureusement).",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "The original definition of S-TAG (Shieber and Schabes, 1990) , however, had a greater generative capacity than that of its component TAG grammars: even though each component grammar could only generate Tree Adjoining Languages (TALs), an S-TAG pairing two TAG grammars could generate non-TALs. Hence, a redefinition was proposed (Shieber, 1994) . Under this new definition, the mapping between grammars occurs at the meta level: there is an isomorphism between derivation trees, preserving structure at the meta level, which establishes the translation. For example, the deriva-\u2022 tion trees for (1) using the elementary trees of Figure 3 is given in Figure 4 ; there is a clear isomorphism, with a bijection between nodes, and parent-child relationships preserved in the mapping.",
"cite_spans": [
{
"start": 33,
"end": 60,
"text": "(Shieber and Schabes, 1990)",
"ref_id": "BIBREF8"
},
{
"start": 329,
"end": 344,
"text": "(Shieber, 1994)",
"ref_id": "BIBREF9"
}
],
"ref_spans": [
{
"start": 629,
"end": 637,
"text": "Figure 3",
"ref_id": null
},
{
"start": 650,
"end": 658,
"text": "Figure 4",
"ref_id": "FIGREF3"
}
],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "In translation, it is not always possible to have a bijection between nodes. Take, for example, In English, hopefully would be represented by a single tree; in French, on esp~re que typically by two. Shieber (1994) proposed the idea of bounded subderivation to deal with such aberrant cases--treating the two nodes in the derivation tree representing on esp~re que as singular, and basing the isomorphism on this. This idea of bounded subderivation solves several difficulties with the isomorphism requirement, but not all. An example by Shieber demonstrates that translation involving clitics causes problems under this definition, as in (3). The partial derivation trees containing the clitic lui and its English parallel are as in Figure 5 .",
"cite_spans": [
{
"start": 200,
"end": 214,
"text": "Shieber (1994)",
"ref_id": "BIBREF9"
}
],
"ref_spans": [
{
"start": 734,
"end": 742,
"text": "Figure 5",
"ref_id": null
}
],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "(3)",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "a. The doctor treats his teeth. b. Le docteur lui soigne les dents.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "A potentially unbounded amount of material intervening in the branches of the righthand tree means that an isomorphism between the trees cannot be established under Shieber's specification even with the modification of bounded subderivations. Shieber suggested that the isomorphism requirement may be overly stringent; Section 3 looks at representing syntactic paraphrase in S-TAG, where similar problems are encountered; in doing this, it can be seen more clearly than in translation that the difficulty is caused not by the isomorphism requirement itself but by the fact that the isomorphism does not exploit any of the structure inherent in the derivation trees.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Machine Translation",
"sec_num": "2"
},
{
"text": "Syntactic paraphrase can also be described with S-TAG (Dras, 1997; Dras, forthcoming) . The manner of representing paraphrase in S-TAG is similar to the translation representation described in Section 2. The reason for illustrating both is that syntactic paraphrase, because of its structural complexity, is able to illuminate the nature of the problem with S-TAG. In a specific parallel, a difficulty like that of the clitics occurs here also, for example in paraphrases such as (4).",
"cite_spans": [
{
"start": 54,
"end": 66,
"text": "(Dras, 1997;",
"ref_id": "BIBREF1"
},
{
"start": 67,
"end": 85,
"text": "Dras, forthcoming)",
"ref_id": null
}
],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "(4) a. The jacket which collected the dust was tweed.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "b. The jacket collected the dust. It was tweed.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "Tree pairs which could represent the elements in the mapping between (4a) and (4b) are given in Figure 6 . It is clearly the case that the trees in the tree pair c~9 are not elementary trees, in the same way that on esp~re que is not represented by a single elementary tree: in both cases, such single elementary trees would violate the Condition on Elementary Tree Minimality (Frank, 1992) . The tree pair a0 is the one that captures the syntactic rearrangement in this paraphrase; such a tree pair will be termed the STRUCTURAL MAPPING PAIR (SMP). Taking as a basic set of trees the XTAG standard grammar of English (XTAG, 1995) , the derivation tree pair for (4) would be as in Figure 7 . 3 Apart from c~9, each tree in Figure 6 corresponds to an elementary object-level tree, as indicated by its label; the remaining labels, indicated in bold in the metalevel' derivation tree in Figure 7 , correspond to the elementary object-level trees forming (~9, in much the same way that on esp~re que is represented by a subderivation comprising an on tree substituted into an esp~re que tree.",
"cite_spans": [
{
"start": 377,
"end": 390,
"text": "(Frank, 1992)",
"ref_id": null
},
{
"start": 618,
"end": 630,
"text": "(XTAG, 1995)",
"ref_id": "BIBREF11"
}
],
"ref_spans": [
{
"start": 96,
"end": 104,
"text": "Figure 6",
"ref_id": null
},
{
"start": 681,
"end": 689,
"text": "Figure 7",
"ref_id": null
},
{
"start": 723,
"end": 731,
"text": "Figure 6",
"ref_id": null
},
{
"start": 884,
"end": 892,
"text": "Figure 7",
"ref_id": null
}
],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "Note that the nodes corresponding to the left tree of the SMP form two discontinuous groups, but these discontinuous groups are clearly related. Dras (forthcoming) describes the conditions under which these discontinuous groupings are acceptable in paraphrase; these discontinuous groupings are treated as a single block with SLOTS connecting the groupings, whose fillers must be of particular types. Fundamentally, however, the structure is the same as for clitics: in one derivation tree the grouped elements are in one branch of the tree, and in the other they are in two separate branches with the possibility of an unbounded amount of intervening material, as described below in Section 4. (5) a. The jacket which collected the dust which covered the floor was tweed.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "b. The jacket which collected the dust 3Node labels, the object-level tree names, are given according to the XTAG standard: see Appendix B of XTAG (1995) . This is done so that the component trees of the aggregate (~9 and their types are obvious. The lexical item to which each is bound is given in square brackets, to make the trees, and the correspondence between for example Figure 6 and Figure 7 The paraphrase in (4) and in Figures 6 and 7 , and other paraphrase examples, strongly suggest that these more complex mappings are not an aberration that can be dealt with by patching measures such as bounded subderivation. It is clear that the meta level is fundamentally not just for establishing a one-to-one onto mapping between nodes; rather, it is also about defining structures representing, for example, the 4The referring expression that is the subject of this second sentence has changed from it in (4) to the dust so the antecedent is clear. Ensuring it is appropriately coreferent, by using two occurrences of the same diacritic in the same tree, necessitates a change in the properties of the formalism unrelated to the one discussed in this paper; see Dras (forthcoming). Assume, for the purpose of this example, that the referring expression is fixed and given, as is the case with it, rather than determined by coindexed diacritics. Figure 8 , it is necessary to regard the SMP components of each tree as a unitary substructure and map them to each other. The discontinuous groupings should form these substructures regardless of intervening material, and this is suggestive of TAG's EDL.",
"cite_spans": [
{
"start": 128,
"end": 153,
"text": "Appendix B of XTAG (1995)",
"ref_id": null
}
],
"ref_spans": [
{
"start": 378,
"end": 386,
"text": "Figure 6",
"ref_id": null
},
{
"start": 391,
"end": 399,
"text": "Figure 7",
"ref_id": null
},
{
"start": 429,
"end": 444,
"text": "Figures 6 and 7",
"ref_id": null
},
{
"start": 1350,
"end": 1358,
"text": "Figure 8",
"ref_id": "FIGREF7"
}
],
"eq_spans": [],
"section": "S-TAG and Paraphrase",
"sec_num": "3"
},
{
"text": "In the TAG definition, the derivation trees are context free (Weir, 1988) , and can be expressed by a CFG. The isomorphism in the S-TAG definition of Shieber (1994) reflects this, by effectively adopting the single-level domain of locality (extended slightly in cases of bounded subderivation, but still effectively a single level), in the way that context free trees are fundamentally made from single level components and grown by concatenation of these single levels. This is what causes the isomorphism requirement to fail, the inability to express substructures at the meta level in order to map between them, rather than just mapping between (effec- To solve the problem with isomorphism, a metalevel grammar can be defined to specify the necessary substructures prior to mapping, with minimality conditions on what can be considered acceptable discontinuity. Specifically, in this case, a TAG meta-level grammar can be defined, rather than the implicit CFG, because this captures the EDL well. The TAG yield function of Weir (1988) can then be applied to these derivation trees to get derived trees. This, of course, raises questions about effects on generative capacity and other properties; these are dealt with in Section 5.",
"cite_spans": [
{
"start": 61,
"end": 73,
"text": "(Weir, 1988)",
"ref_id": "BIBREF10"
},
{
"start": 1027,
"end": 1038,
"text": "Weir (1988)",
"ref_id": "BIBREF10"
}
],
"ref_spans": [],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "A procedure for automatically constructing a TAG meta-grammar is as follows in Construction 1. The basic idea is that where the node bijection is still appropriate, the grammar retains its context free nature (by using singlelevel TAG trees composed by substitution, mimicking CFG tree concatenation), but where EDL is required, multi-level TAG initial trees are defined, with TAG auxiliary trees for describing the intervening material. These meta-level trees are then mapped appropriately; this corresponds to a bijection of nodes at the metameta level. For (5), the meta-level grammar for the left projection then looks as in Figure 9 , and for the right projection as in Figure 10 . \u2022 Figure 11 contains the meta-meta-level trees, the tree pair that is the derivation of the meta level, where the mapping is a bijection between nodes. Adding unbounded material would then just be reflected in the meta-meta-level as a list of/3 nodes depending from the j315/j31s nodes in these trees.",
"cite_spans": [],
"ref_spans": [
{
"start": 629,
"end": 637,
"text": "Figure 9",
"ref_id": "FIGREF8"
},
{
"start": 675,
"end": 684,
"text": "Figure 10",
"ref_id": null
},
{
"start": 689,
"end": 698,
"text": "Figure 11",
"ref_id": "FIGREF9"
}
],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "The question may be asked, Why isn't it the case that the same effect will occur at the metameta level that required the meta-grammar in the first place, leading perhaps to an infinite (and useless) sequence? The intuition is that it is the meta-level, rather than anywhere 'higher', which is fundamentally the place to specify structure: the object level specifies the trees, and the meta level specifies the grouping or structure of these trees. Then the mapping takes place on these structures, rather than the object-level trees; hence the need for a grammar at the meta-level but not beyond.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "Construction 1 To build a TAG metagrammar:",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "1. An initial tree in the metagrammar is formed for each part of the derivation tree corresponding to the substructure representing an SMP, including the slots so that a contiguous tree is formed. Any node that links these parts of the derivation tree to other subtrees in the derivation tree is also included, and becomes a substitution node in the metagrammar tree.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "2. Auxiliary trees are formed corresponding to the parts of the derivation trees that are slot fillers along with the nodes in the discontinuous regions adjacent to the slots; one contiguous auxiliary tree is formed for each bounded sequence of slot fillers within each substructure. These trees also satisfy certain minimality conditions. The minimality conditions in Step 2 of Construction 1 are in keeping with the idea of minimality elsewhere in TAG (for example, Frank, 1992) . The key condition is that meta-level auxiliary trees are rooted in c~-labelled nodes, and have only ~-labelled nodes along the spine. The intuition here is that slots (the nodes which meta-level auxiliary trees adjoin into) must be c~-labelled: fl-labelled trees would not need slots, as the substructure could instead be continuous and the j3-1abelled trees would just adjoin in. So the meta-level auxiliary trees are rooted in c~-labelled trees; but they have only ~labelled trees in the spine, as they aim to represent the minimal amount of recursive material. Notwithstanding these conditions, the construction is quite straightforward.",
"cite_spans": [
{
"start": 468,
"end": 480,
"text": "Frank, 1992)",
"ref_id": null
}
],
"ref_spans": [],
"eq_spans": [],
"section": "SMP at this meta level: in an isomorphism between trees in",
"sec_num": null
},
{
"text": "Weir (1988) showed that there is an infinite progression of TAG-related formalisms, in generative capacity between CFGs and indexed grammars. A formalism ~-i in the progression is defined by applying the TAG yield function to a derivation tree defined by a grammar formalism 5~i_1 ; the generative capacity of ~i is a superset of ~'i-1-Thus using a TAG meta-grammar, as described in Section 4, would suggest that the generative capacity of the object-level formalism would necessarily have been increased over that of TAG.",
"cite_spans": [
{
"start": 5,
"end": 11,
"text": "(1988)",
"ref_id": null
}
],
"ref_spans": [
{
"start": 275,
"end": 280,
"text": "5~i_1",
"ref_id": null
}
],
"eq_spans": [],
"section": "Generative Capacity",
"sec_num": "5"
},
{
"text": "However, there is a regular form for TAGs (Rogers, 1994) , such that the trees of TAGs in this regular form are local sets; that is, they are context free. The meta-level TAG built by Construction 1 with the appropriate conditions on slots is in this regular form. A proof of this is in Dras (forthcoming); a sketch is as follows. If adjunction may not occur along the spine of another auxiliary tree, the grammar is in regular form. This kind of adjunction does not occur under Construction 1 because all meta-level auxiliary trees are rooted in c~-labelled trees (object-level auxiliary trees), while their spines consist only of p-labelled trees (object-level initial trees).",
"cite_spans": [
{
"start": 42,
"end": 56,
"text": "(Rogers, 1994)",
"ref_id": null
}
],
"ref_spans": [],
"eq_spans": [],
"section": "Generative Capacity",
"sec_num": "5"
},
{
"text": "Since the meta-level grammar is context free, despite being expressed using a TAG grammar, this means that the object-level grammar is still 8{} a TAG.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "Generative Capacity",
"sec_num": "5"
},
{
"text": "In principle, a meta-grammar is desirable, as it specifies substructures at a meta level, which is necessary when operations are carried out that are applied at this meta level. In a practical application, it solves problems in one such formalism, S-TAG, when used for paraphrase or translation, as outlined by Shieber (1994) . Moreover, the formalism remains fundamentally the same, in specifying mappings between two grammars of restricted generative capacity; and in cases where this is important, it is possible to avoid changing the generative capacity of the S-TAG formalism in applying this meta-grammar.",
"cite_spans": [
{
"start": 311,
"end": 325,
"text": "Shieber (1994)",
"ref_id": "BIBREF9"
}
],
"ref_spans": [],
"eq_spans": [],
"section": "Conclusion",
"sec_num": "6"
},
{
"text": "Currently this revised version of the S-TAG formalism is used as the low-level representation in the Reluctant Paraphrasing framework of Dras (1998; forthcoming) . It is likely to also be useful in representations for machine translation between languages that are structurally more dissimilar than English and French, and hence more in need of structural definition of objectlevel constructs; exploring this is future work.",
"cite_spans": [
{
"start": 137,
"end": 148,
"text": "Dras (1998;",
"ref_id": "BIBREF2"
},
{
"start": 149,
"end": 161,
"text": "forthcoming)",
"ref_id": null
}
],
"ref_spans": [],
"eq_spans": [],
"section": "Conclusion",
"sec_num": "6"
},
{
"text": "2In derivation trees, addresses are given using the Gorn addressing scheme, although these are omitted in this paper where the composition operations are obvious.",
"cite_spans": [],
"ref_spans": [],
"eq_spans": [],
"section": "",
"sec_num": null
}
],
"back_matter": [],
"bib_entries": {
"BIBREF0": {
"ref_id": "b0",
"title": "Using Lexicalized TAGs for Machine Translation",
"authors": [
{
"first": "@",
"middle": [],
"last": "Abeill",
"suffix": ""
},
{
"first": "Anne",
"middle": [],
"last": "",
"suffix": ""
},
{
"first": "Yves",
"middle": [],
"last": "Schabes",
"suffix": ""
},
{
"first": "Aravind",
"middle": [],
"last": "Joshi",
"suffix": ""
}
],
"year": 1990,
"venue": "Proceedings of the 13th International Conference on Computational Linguistics",
"volume": "",
"issue": "",
"pages": "1--6",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Abeill@, Anne, Yves Schabes and Aravind Joshi. 1990. Using Lexicalized TAGs for Machine Trans- lation. Proceedings of the 13th International Con- ference on Computational Linguistics, 1-6.",
"links": null
},
"BIBREF1": {
"ref_id": "b1",
"title": "Representing Paraphrases Using S-TAGs",
"authors": [
{
"first": "Mark",
"middle": [],
"last": "Dras",
"suffix": ""
}
],
"year": 1997,
"venue": "Proceedings of the 35th Meeting of the Association for Computational Linguistics",
"volume": "",
"issue": "",
"pages": "516--518",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Dras, Mark. 1997. Representing Paraphrases Using S-TAGs. Proceedings of the 35th Meeting of the As- sociation for Computational Linguistics, 516-518.",
"links": null
},
"BIBREF2": {
"ref_id": "b2",
"title": "Search in Constraint-Based Paraphrasing",
"authors": [
{
"first": "Mark",
"middle": [],
"last": "Dras",
"suffix": ""
}
],
"year": 1998,
"venue": "Natural Language Processing and Industrial Applications (NLPq-IA98)",
"volume": "",
"issue": "",
"pages": "213--219",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Dras, Mark. 1998. Search in Constraint-Based Paraphrasing. Natural Language Processing and In- dustrial Applications (NLPq-IA98), 213-219.",
"links": null
},
"BIBREF3": {
"ref_id": "b3",
"title": "Tree Adjoining Grammar and the Reluctant Paraphrasing of Text",
"authors": [
{
"first": "Mark",
"middle": [],
"last": "Dras",
"suffix": ""
}
],
"year": null,
"venue": "",
"volume": "",
"issue": "",
"pages": "",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Dras, Mark. forthcoming. Tree Adjoining Grammar and the Reluctant Paraphrasing of Text. PhD thesis, Macquarie University, Australia.",
"links": null
},
"BIBREF4": {
"ref_id": "b4",
"title": "Pseudo-Projectivity: A Polynomially Parsable Non-Projective Dependency Grammar",
"authors": [
{
"first": "Aravind",
"middle": [],
"last": "Joshi",
"suffix": ""
},
{
"first": "Yves",
"middle": [],
"last": "Schabes",
"suffix": ""
}
],
"year": 1996,
"venue": "Proceedings of the 36th Annual Meeting of the Association for Computational Linguistics",
"volume": "3",
"issue": "",
"pages": "646--652",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Joshi, Aravind and Yves Schabes. 1996. Tree- Adjoining Grammars. In Grzegorz Rozenberg and \u2022 Arto Salomaa (eds.), Handbook of Formal Lan- guages, Vol 3, 69-123. Springer-Verlag. New York, NY. Kahane, Sylvain, Alexis Nasr and Owen Ram- bow. 1998. Pseudo-Projectivity: A Polynomi- ally Parsable Non-Projective Dependency Gram- mar. Proceedings of the 36th Annual Meeting of the Association for Computational Linguistics, 646-652.",
"links": null
},
"BIBREF5": {
"ref_id": "b5",
"title": "Rapid Prototyping of Domain-Specific Machine Translation Systems",
"authors": [
{
"first": "Martha",
"middle": [],
"last": "Palmer",
"suffix": ""
},
{
"first": "Owen",
"middle": [],
"last": "Rainbow",
"suffix": ""
},
{
"first": "Alexis",
"middle": [],
"last": "Nasr",
"suffix": ""
}
],
"year": 1994,
"venue": "Proceedings of the 32nd Meeting of the Association for Computational Linguistics",
"volume": "",
"issue": "",
"pages": "155--162",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Palmer, Martha, Owen Rainbow and Alexis Nasr. 1998. Rapid Prototyping of Domain-Specific Ma- chine Translation Systems. AMTA-98, Langhorne, PA. Rogers, James. 1994. Capturing CFLs with Tree Adjoining Grammars. Proceedings of the 32nd Meet- ing of the Association for Computational Linguis- tics, 155-162.",
"links": null
},
"BIBREF6": {
"ref_id": "b6",
"title": "An Alternative Conception of Tree-Adjoining Derivation",
"authors": [
{
"first": "Yves",
"middle": [],
"last": "Schabes",
"suffix": ""
},
{
"first": "Stuart",
"middle": [],
"last": "Shieber",
"suffix": ""
}
],
"year": 1994,
"venue": "Computational Linguistics",
"volume": "20",
"issue": "1",
"pages": "91--124",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Schabes, Yves and Stuart Shieber. 1994. An Al- ternative Conception of Tree-Adjoining Derivation. Computational Linguistics, 20(1): 91-124.",
"links": null
},
"BIBREF7": {
"ref_id": "b7",
"title": "Evidence against the contextfreeness of natural language",
"authors": [
{
"first": "Stuart",
"middle": [],
"last": "Shieber",
"suffix": ""
}
],
"year": 1985,
"venue": "Linguistics and Philosophy",
"volume": "8",
"issue": "",
"pages": "333--343",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Shieber, Stuart. 1985. Evidence against the context- freeness of natural language. Linguistics and Philos- ophy, 8, 333-343.",
"links": null
},
"BIBREF8": {
"ref_id": "b8",
"title": "Synchronous Tree-Adjoining Grammars",
"authors": [
{
"first": "Stuart",
"middle": [],
"last": "Shieber",
"suffix": ""
},
{
"first": "Yves",
"middle": [],
"last": "Schabes",
"suffix": ""
}
],
"year": 1990,
"venue": "Proceedings of the 13th International Conference on Computational Linguistics",
"volume": "",
"issue": "",
"pages": "253--258",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Shieber, Stuart and Yves Schabes. 1990. Syn- chronous Tree-Adjoining Grammars. Proceedings of the 13th International Conference on Computational Linguistics, 253-258.",
"links": null
},
"BIBREF9": {
"ref_id": "b9",
"title": "Restricting the Weak-Generative Capacity of Synchronous Tree-Adjoining Grammars",
"authors": [
{
"first": "Stuart",
"middle": [],
"last": "Shieber",
"suffix": ""
}
],
"year": 1994,
"venue": "Computational Intelligence",
"volume": "10",
"issue": "4",
"pages": "371--386",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Shieber, Stuart. 1994. Restricting the Weak- Generative Capacity of Synchronous Tree-Adjoining Grammars. Computational Intelligence, 10(4), 371- 386.",
"links": null
},
"BIBREF10": {
"ref_id": "b10",
"title": "Characterizing Mildly Context-Sensitive Grammar Formalisms",
"authors": [
{
"first": "David",
"middle": [],
"last": "Weir",
"suffix": ""
}
],
"year": 1988,
"venue": "",
"volume": "",
"issue": "",
"pages": "",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "Weir, David. 1988. Characterizing Mildly Context- Sensitive Grammar Formalisms. PhD thesis, Uni- versity of Pennsylvania.",
"links": null
},
"BIBREF11": {
"ref_id": "b11",
"title": "A Lexicalized Tree Adjoining Grammar for English",
"authors": [
{
"first": "",
"middle": [],
"last": "Xtag",
"suffix": ""
}
],
"year": 1995,
"venue": "",
"volume": "",
"issue": "",
"pages": "",
"other_ids": {},
"num": null,
"urls": [],
"raw_text": "XTAG. 1995. A Lexicalized Tree Adjoining Gram- mar for English. Technical Report IRCS95-03, Uni- versity of Pennsylvania.",
"links": null
}
},
"ref_entries": {
"FIGREF0": {
"uris": null,
"num": null,
"text": "Figure 1: Elementary TAG trees",
"type_str": "figure"
},
"FIGREF1": {
"uris": null,
"num": null,
"text": "Derived and derivation trees, respectively, forFigure 1",
"type_str": "figure"
},
"FIGREF2": {
"uris": null,
"num": null,
"text": "misses Mary. b. Marie manque g Jean.",
"type_str": "figure"
},
"FIGREF3": {
"uris": null,
"num": null,
"text": "Derivation tree pair for Fig 3 (2) a. Hopefully John misses Mary. b. On esp~re que Marie manque Jean.",
"type_str": "figure"
},
"FIGREF4": {
"uris": null,
"num": null,
"text": "illustrates why the paraphrase in (4) has the same difficulty as the clitic example in (3) when represented in S-TAG: because unbounded intervening material can occur when promoting arbitrarily deeply embedded relative clauses to sentence level, as indicated byFigure 8,an isomorphism is not possible between derivation trees representing paraphrases such as (4) and (5). Again, the component trees of the SMP are in bold inFigure 8.",
"type_str": "figure"
},
"FIGREF5": {
"uris": null,
"num": null,
"text": "Figure 6: S-TAG for (4)",
"type_str": "figure"
},
"FIGREF6": {
"uris": null,
"num": null,
"text": "Figure 7: Derivation tree pair for example (4)",
"type_str": "figure"
},
"FIGREF7": {
"uris": null,
"num": null,
"text": "Derivation tree for example(5)tively) single nodes.",
"type_str": "figure"
},
"FIGREF8": {
"uris": null,
"num": null,
"text": "these single-level trees in the metagrammar marked for substitution if the corresponding nodes in the derivation tree have subtrees.",
"type_str": "figure"
},
"FIGREF9": {
"uris": null,
"num": null,
"text": "Derivation tree pair for Fig 3",
"type_str": "figure"
},
"TABREF5": {
"html": null,
"num": null,
"type_str": "table",
"content": "<table><tr><td/><td/><td/><td>flVvx[~s]</td><td colspan=\"2\">.. _ %~xdx~lNf~c~</td><td>~Vvx[is]</td><td>/~sPUs[.]</td></tr><tr><td>~DXDI, h0]</td><td colspan=\"2\">~l[:o~I~dJ</td><td colspan=\"2\">~DXD[the]</td><td>~N0nx0Vnx l[coliect ed]</td><td>anxOVnxl [covered]</td></tr><tr><td>/~COMPs[which]</td><td/><td>aNXdxN[dust]</td><td/><td/><td>~COMPs[which] aNXdxN[dust]</td><td>aNXN[it] oNXdxN[floor]</td></tr><tr><td/><td>aDXD[the]</td><td>/~N0nx0Vnxl [covered]</td><td/><td/><td>~DXD[the]</td><td>aDXD[the]</td></tr><tr><td/><td/><td>aDXD[t he]</td><td/><td/></tr></table>",
"text": "............... y Nx\u00a2~]"
}
}
}
} |