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Why TEI stand-off annotation doesn't quite work

and why you might want to use it nevertheless

Piotr Bański

Assistant Professor

Institute of English Studies, University of Warsaw

Balisage: The Markup Conference 2010
August 3 - 6, 2010

Copyright © 2010 by the author. Used with permission.

How to cite this paper

Bański, Piotr. “Why TEI stand-off annotation doesn't quite work: and why you might want to use it nevertheless.” Presented at Balisage: The Markup Conference 2010, Montréal, Canada, August 3 - 6, 2010. In Proceedings of Balisage: The Markup Conference 2010. Balisage Series on Markup Technologies, vol. 5 (2010). doi:10.4242/BalisageVol5.Banski01.

Abstract

The present submission focuses on the concept of stand-off annotation as it is implemented in the current version of the TEI Guidelines. We look at the motivation for choosing the stand-off approach to encoding Language Resources, briefly recount the history of the concept within the broadly conceived TEI setting (since TEI P3 and the LT NSL suite, through CES and XCES, ending in TEI P5), review the various kinds of hyperlink semantics and identify three kinds of reasons for the poor uptake of the TEI-recommended stand-off annotation approach to corpus encoding. We also suggest some solutions that may contribute to a change in the current state of affairs.

Table of Contents

1. Introduction
2. Practical motivation for stand-off representations
2.1. OHCO, overlap, modularity, and the nature of OWLs
2.2. Issues of sustainability and interoperability
2.3. Selected TEI stand-off systems
2.4. Motivation for stand-off representation: summary
3. Stand-off annotation: the semantics of hyperlinks
4. Stand-off markup in the TEI
4.1. local stand-off
4.2. The converging paths of the XCES and TEI P5
5. Problems with implementing TEI stand-off annotation
5.1. Technical issues external to the TEI
5.2. TEI-internal technical issues
5.3. Social and sociological issues
6. A sketch of solutions
7. Conclusion
Acknowledgements

1. Introduction

The present contribution concerns the application of the TEI Guidelines ([TEI Consortium, 2010]) to the description of Language Resources (LRs), defined as follows:

A Language Resource is any physical or digital item that is a product of language documentation, description, or development, or is a tool that specifically supports the creation and use of such products ([Simons & Bird, 2008]).

More specifically, we are looking at linguistic corpora – what [Witt et al., 2009a] call static text-based LRs. We furthermore restrict the discussion to text corpora, though we believe that much of it is true of e.g. speech corpora, or multimodal corpora in general. We also believe that the general implications of the discussion can be carried over to other places where linguistics meets markup, or, more generally still, where two communities with different backgrounds meet to describe an range of phenomena of interest to both of them.

Among text corpora, we look at those encoded in TEI XML. [1] The aim of the paper is to assess the suitability of the TEI for the purpose of creating multi-layer descriptions of linguistic phenomena, but also for more focused applications, such as those described in [Boot, 2009] or [Cummings, 2009].

The question we will ask is not whether stand-off annotation in the TEI is doable – the answer to that is clear and successful stand-off TEI systems exist. The question will be rather: is stand-off TEI feasible, available out-of-the-box to an XML-literate OWL (Ordinary Working Linguist) with a crush for the TEI.[2] Here, the answer is unfortunately, it depends, and we shall look at the dependencies. Some of them are internal to the TEI and hence potentially open to relatively quick local fixing, some of them external and affecting the XML world at large. The TEI-internal issues will be shown to have twofold nature, technological and sociological, the former easier to solve and conditioning the latter.

The discussion is based on the author's experience in setting up stand-off TEI architecture in the National Corpus of Polish (a 109-word resource nearing completion, cf. [Bański & Przepiórkowski, 2010]), the Open-Content Text Corpus (at the alpha stage, cf. [Bański & Wójtowicz, 2010]), and the Foreign Language Examination Corpus (at the planning stage, cf. [Bański & Gozdawa-Gołębiowski, 2010]).

In what follows, we first attempt to answer the question of why bother: why stand-off markup is an attractive technique from the point of view of a linguist (Section 2). Next, in Section 3, we briefly look at the history of SGML and XML stand-off approaches in the broadly defined context of the TEI and also at the semantics postulated for the interpretation of stand-off devices. In Section 4, we look at the TEI's approach to stand-off annotation, and in Section 5 at the various issues that may condition the insufficient level of uptake of this approach in the linguistic community. Finally, in Section 6, we sketch some solutions for the problems identified in the present article. Section 7 concludes the paper.

2. Practical motivation for stand-off representations

This section looks at the motivation for using stand-off representations as seen from the point of view of an Ordinary Working Linguist. The arguments come mostly from modularity, both theoretical and practical, but we also look at the issues of sustainability and interoperability of LRs. We finish by presenting three different multi-layer TEI stand-off annotation systems as illustration and a point of reference for further discussion.

2.1. OHCO, overlap, modularity, and the nature of OWLs

One of the claims that gave markup studies a solid push was the thesis that text is OHCO, an ordered hierarchy of content objects[3] . The thesis has been shown to be both inaccurate as a general claim and valid as a statement of tendencies and pragmatic advantages: while the OHCO thesis does not hold in all cases, due to the existence of overlapping hierarchies and non-contiguous objects, it appears to constrain many conceptualizations of the nature of text, and OHCO-based approaches to e.g. text editing appear to have practical advantages. Much of linguistic modelling is also done assuming OHCO as the general conceptual approach, accompanied by additional devices (movement, linking, feature percolation, re-entrancy, etc.) as ways to more or less system(at)ically plug the holes that OHCO alone cannot fill.

Criticism of the OHCO thesis has appeared extensively in the literature – see e.g. [Renear et al., 1993] for an early formulation of the problems and reformulations of the thesis, and [DeRose, 2004] for an overview of ways in which non-OHCO structures can be represented, also within the TEI; the Extreme Markup Languages and Balisage series contain numerous articles devoted to this issue. Our purpose here is not to provide new flashy arguments for something that has already sprung extensive research on alternatives to XML and on ways to handle the failure of the OHCO thesis by devices native to XML. Our aim is practical: we point out that overlap and discontinuity, and the need to embrace rather than trick them, are inherent in both theoretical linguistic constructs and in corpus linguistic practice. We furthermore point out that the existence of mismatches in description is one of the arguments for a modular approach to linguistic modelling, whereby objects with sometimes strikingly different properties are supposed to constitute separate domains of study, which are linked by correspondence or mapping rules. This is what we mean by theoretical modularity. There is also a more practical aspect of modularity, where it is advisable to keep the output of various linguistic tools separated, especially where each of these separate outputs may constitute the base for further descriptions in a multi-layer system.

This state of affairs is not only due to the fact that different kinds of linguistic description require different and often conflicting segmentations at various levels, some examples of which we shall look at below. It is also due to the fact that there is no single way to demarcate the domain of any component of grammar – there are a multitude of syntactic, semantic, morphological, phonological, etc. theories with differing theoretical apparatus, and sometimes even with differing domains of application, although they are theories of seemingly the same phenomena. Consider the virtual non-existence of linguistic morphology in the days of the early Generative Grammar (from the late 50's throughout the 60's), when the syntagmatic aspect of word composition (ordering of morphs, the "atoms of word forms") was delegated to the syntactic component, and its paradigmatic aspect (allomorphy, i.e. modifications in the shape of morphs) was delegated to the ultra-powerful phonological component (cf. [Anderson, 1992], ch. 2 for a concise discussion and references). Consider also the lack of interest of Classical Phonemics in morphophonological phenomena[4], which later became part of the focus of Generative Phonology. Similar remarks concern the division of labour between and across the semantic and pragmatic components – for those models that distinguish between the two – vis-à-vis models based on so-called Cognitive Grammar, which introduce different divisions. The point is that there is no single unified approach to morphology, syntax or semantics, etc., and any encoding strategy choosing one particular perspective as privileged is bound to attract criticism and to discourage researchers working in different paradigms. We OWLs can sometimes agree that what we want to describe is stretches of manifestations of natural language. Sometimes, this is also the limit of our consent, and anything beyond this, e.g. our views on the proper segmentation of these stretches, should be presented as equal variants rather than one "proper" version with possible "deviations".

On a plane more familiar to markup specialists, consider an example from [Wörner et al., 2006] that concisely presents the nature of the problem with overlapping linguistic hierarchies: the French preposition de and article la are pronounced as a single phonological unit, [dla]. At the same time, the preposition and the article are children of two different nodes in a syntactic tree:

Figure 1: Overlapping lexical, phonological and syntactic hierarchies (copied from [Wörner et al., 2006])

image ../../../vol5/graphics/Banski01/Banski01-001.png

The same is true of other cases involving separate syntactic elements getting merged morphologically or prosodically, as in the German in das becoming ins, English gonna , won't or I'd've, the last of which is a fairly typical example of cliticization (where a syntactically independent element is prosodically dependent on another; in this case, both the contracted 'd and 've cliticize onto the pronoun I), and of numerous other examples cited in the linguistic literature, often under the heading of "bracketing paradoxes" or "mismatches" of various sorts.

Consider also somewhat different misalignments, for example conflicting POS (part-of-speech) descriptions. These may involve changes in the number and the kind of grammatical labels used (e.g. compare the various tagsets of the CLAWS tagger), but the differences may in many cases go deeper and may involve conflicting segmentations: compare the divisions within [does][n't] (CLAWS/Penn Treebank) vs. [doesn]['][t] (TnT Tagger), as adduced by [Chiarcos et al., 2009].[5] Some relevant cases are illustrated below.

Figure 2: Conflicting tokenizations: morpholexical (of the English doesn't and the Polish goście) and syntactic (się-haplology in Polish)

image ../../../vol5/graphics/Banski01/Banski01-002.png

On the left, we present two attested strategies for the tokenization of the English doesn't (after [Chiarcos et al., 2009]). In the middle, two possibilities of the interpretation of the string goście in Polish are shown, whereas the diagram on the right illustrates overlapping syntactic segments, where obawiał się means "he was afraid" and się uśmiechnąć means "to smile"; notice that both strings involve the reflexive marker się, preposed with respect to the second verb's lemma (= canonical form).

While the first case demonstrates a single function word with multiple possible segmentations depending on the given software tool, case (b) shows a single form that realizes distinct "underlying" sequences: either a plural noun (consisting of a stem and an ending (desinence) – but this level of detail is rarely needed) or a weak pronoun go "him" followed by an auxiliary (person-number) clitic śmy. Case (c) shows two overlapping syntactic words – this is an example of the haplology of the Polish reflexive marker (see [Kupść, 1999]). The marker is obligatory for both verbs used here (the forms *obawiał and *uśmiechnął are ungrammatical without the accompanying się) but under appropriate circumstances, multiple instances of się may (and in fact should, in idiomatic Polish) reduce to a single occurrence that is perceived as shared by the verbs involved. (As a further complication, these parts of the reflexive verb need not be adjacent.)

Although all of the examples above present various cases of overlap, we do not want to treat them in the same way. Cases (b) and (c) belong to the same respective levels of grammatical description (basic segmentation in (b), syntactic word identification in (c)) and the contrast between the alternatives in each case is not based on any theoretical difference – in the words of [Renear et al., 1993], they belong to a single perspective, and therefore are a counterexample to even the weakest version of OHCO. At the same time, we do not want to subject them to any kind of non-OHCO mechanism apart from a simple disjunction between (sub)trees: we want a single document to provide us with both variant readings in the case of (b), and both syntactic words in the case of (c). Example (a), on the other hand, may be argued to show different perspectives, as defined from the point of view of the software tool that is used to tokenize and tag the resulting strings. In such cases, we want the different tokenizations to reside in different documents.[6]

Similarly, if the only difference lies in the assignment of POS labels – for example, the tag for the comparative degree of an adjective (better, older) in the CLAWS-5 tagset used to tag the British National Corpus, is "AJC", whereas in the CLAWS-8 tagset it is "JJR" – then, although expressing the labels in a single document would be trivial (e.g. in multi-valued attributes), we want them placed in separate documents, because they represent different perspectives or at least different tools. This is completely independent from the practical issue of validation of such multi-token attributes and the like – even if the validation were trivial, these perspectives are fundamentally different for practical reasons and should be kept separate also with an eye to using one of them but not the other for the purpose of building the next annotation layer. [7]

Consider one more example, of the ambiguous sentence they killed the man with an umbrella. The realistic phrase structure analysis in such cases stops at the level of chunking (shallow parsing) – in this case, [they][killed][the man][with the umbrella], with no indication of the structure of the verb phrase (VP) that starts at kill and continues to the end of the sentence on either reading. If deep parsing were attempted, the result would be as in (a) below, where the prepositional phrase modifies either the verb kill (upper tree) or the noun man (lower tree).

Figure 3: Conflicting phrase structure analyses of a single sentence (a) vs. dependency analyses (b) and (c)

image ../../../vol5/graphics/Banski01/Banski01-003.png

The prepositional phrase (PP) with an umbrella can be interpreted either as a separate instrumental adverbial (upper tree in (a)) or as part of the noun phrase (NP) object (a modifier of the noun – not indicated separately in the lower tree in (a)). In the dependency analysis, we are looking at graphs with labelled edges. The meanings of the labels are as follows: main introduces the entire structure, subj = subject, obj = object, instr = instrumental adverbial, det = determiner (article), pcomp = prepositional complement; diagram (c) shows the modification of the object man necessary to reflect the interpretation whereby the man was carrying an umbrella (=the lower tree in (a)), mod = modifier.

Examples (b) and (c) represent a dependency analysis of the same sentence (based on the http://www.connexor.eu/ "machinese" demo), with (b) corresponding to the interpretation encoded by the top tree in (a), while (c) reflects the interpretation of the bottom tree of (a). Dependency analyses involve graphs with labelled edges, which may – but need not – be isomorphic with trees, and therefore their OHCO-compliance can at best be partial.

In essence, what is needed for linguistic description is best captured by keeping the text in as neutral form as possible, and offering various views of it, depending on the whim or the particular set of linguistic beliefs of the given user. One way to achieve this goal is to use stand-off annotation, whereby the source data is kept separate, either as raw text or with “low density” XML markup (i.e., with gross structural markup alone, e.g. identifying headers and paragraphs but little more, in order not to instil any theoretical linguistic interpretation into the text), and whereby all the possible linguistic interpretations are kept in separate documents, either referencing the source text directly, or forming a hierarchy of annotation layers (see e.g. [Goecke et al., 2010] or [Ide & Romary, 2007] for more details).[8]

While some of the cases of overlap and discontinuity presented here are open to reanalysis in terms other than stand-off annotation, even in the TEI itself – by means of milestone elements, fragmentation, in-file stand-off elements such as <link> and <join> or linking attributes such as @exclude, @synch and others (cf. [DeRose, 2004] and chapters 16 and 20 of the TEI Guidelines), there is one important factor that rules out such strategies, and that is modularity of description. Descriptions of properties belonging to different theoretical perspectives are expected to be separate, in order to constitute separate modules that can be judged, verified and challenged on their own.[9]

2.2. Issues of sustainability and interoperability

It is one of the tenets of at least some sustainability-oriented encoding practices that the object of description (in our case, text) be maximally divorced from its possible theoretical views (annotations). This way, the text, kept in as neutral form as possible, remains an attractive resource, open to future analyses and to the creation of new views, i.e., new annotation layers (this goes under the heading of extensibility). Equally attractive are the annotations themselves – they can serve as the basis for comparison of tools and theories. Security (immutability) of the text itself is also essential, and this is what stand-off approaches strive to guarantee, because they are non-destructive with respect to the resource that gets annotated.[10].

Interoperability values the ease of transduction, both in the case of the source text and with respect to its annotations. Sometimes, the ease of mapping a single layer of annotation to another resource (e.g. a translated document) is also important.

On the other hand, [Rehm et al., 2010] point out that stand-off approaches are not optimal from the point of view of sustainability because they require dedicated tools in order to merge annotations with the source text. This is very true of the current state of affairs. Our point is that if stand-off annotation can be handled by generic XML tools then the issue of the longevity of the annotation layers (note that the source text is relatively safe) piggybacks on the general well-being of XML technology, ages together with it, and is open to whatever plastic surgery is applied to make XML or its descendants look good 20 years from now.

[Rehm et al., 2010] point out that the approach they suggest, multiply-annotated text, which also uses layers of annotation but each of these layers contains an exact copy of the source text, and thus achieves sustainability through redundancy, has more advantages than stand-off approaches that keep a single copy of the source text. It is not our aim to argue against that theoretical stance because, like the stand-off approach that we concentrate on here, it assumes modularity of description, and modularity is what OWLs need. Additionally, in principle, both approaches can be mixed in e.g. crowd-sourced corpora where annotation layers are contributed by external parties. Both approaches also appear to share one more problem: the lack of generic XML tool support, a matter which we will return to below.

Summing up, stand-off technology has both advantages and disadvantages from the point of view of the two deservedly hot leitmotifs of language documentation and linguistic infrastructure: sustainability and interoperability. On the one hand, the advocates of stand-off markup note the relative stability of source text with low-density markup (or with no markup at all), as well as the putative flexibility of the annotation layers. On the other hand, those who concentrate on the holistic advantages of language resources note that the merger of the source with the annotation layers requires dedicated machinery. In the next section, we look at three stand-off TEI systems that attempt to cope with these issues in various ways.

2.3. Selected TEI stand-off systems

The present section contains brief descriptions of selected complex systems involving versions of the TEI stand-off technology. The selection is absolutely partial and subjective, but, we believe, it serves its purpose nevertheless, exemplifying three out of many possible variants of stand-off systems.

The first resource to be presented is the National Corpus of Polish (NCP), an over-109-segment deliverable of a 3-year state-funded project ending in late 2010, available for searching at http://nkjp.pl/. We present the structure of a single corpus text in the diagram below.

Figure 4: National Corpus of Polish (NCP): dependencies in a robust multi-layer stand-off system

image ../../../vol5/graphics/Banski01/Banski01-004.png

Dependencies among the annotation layers in the National Corpus of Polish. Red arrows (image ../../../vol5/graphics/Banski01/Banski01-005.png ) denote the dependencies among the various parts of the hierarchy. Blue arrows (image ../../../vol5/graphics/Banski01/Banski01-006.png) and purple arrows (image ../../../vol5/graphics/Banski01/Banski01-007.png) signal the inclusion of the local header and the main corpus header, respectively. For the sake of readability, these relationships are not indicated in the diagrams that follow.

In the NCP, the source text has minimal structural inline markup, down to the level of the paragraph (<p> or <ab>, the latter standing for "anonymous block" where we don't want to make a semantic commitment). Sentence boundaries as well as the individual tokens are identified at the segmentation layer (1.); this is also where segmental ambiguities such as those discussed in Figure 2 (b) are indicated. The segmentation layer serves as the basis for the layer that, firstly, identifies all the morphological interpretations of the given segment, and secondly, attempts to disambiguate them in the morphosyntactic context (2.); this layer is referenced by the next two: (3.) the layer of syntactic words (grouping e.g. analytic tense realizations but also elements such as obawiać się and uśmiechnąć się, cf. Figure 2 (c)) and (4.) the layer of word-sense disambiguation (experimental, for 100 selected lexemes with multiple interpretations). The layer of syntactic words is the basis for the final two layers: the layer of named-entity recognition (5.)[11] and the layer of shallow parsing, identifying syntactic chunks (6.). All NCP documents, source text and annotations alike, include two kinds of headers: the local header that describes the properties of the source text and contains a changelog for all the modifications and additions that affect the given directory, and the single corpus header, which contains information shared by all parts of the corpus, including definitions of various taxonomies, which are referenced from the local headers. An early version of TEI ODD "literate encoding" documents describing some of these schemas has been made available at http://nlp.ipipan.waw.pl/TEI4NKJP/. See [Bański & Przepiórkowski, 2010] for more description and references to more detailed papers on each of the annotation layers.

The two resources that follow took the overall model of the NCP as their starting point, and tailored it to their specific purposes and the context in which they are deployed. The first of them is the Open-Content Text Corpus. This is a resource meant to be both the open-source testing ground for TEI stand-off applications and at the same time, to constitute a common platform for collective research and academic work on preserving and describing language resources, especially those for "lower-density languages". While we leave the details aside (see [Bański & Wójtowicz, 2010]), we note that the multilingual nature of the corpus (at the time of writing, it contains mini-subcorpora for 55 languages) forces the introduction of one more layer of organization, with its own header. Thus, each text of the OCTC includes three headers, the links to which have been mercifully omitted from the diagram below. Because the corpus is meant as a platform for many possible research or student teams, it is explicitly modelled as a multi-instance stand-off structure, which means that it is expected that a single annotation layer of the OCTC may come in many variants, depending on the tools used to create it. This is indicated below.

Figure 5: Open-Content Text Corpus (OCTC): dependencies in a multi-instance stand-off system

image ../../../vol5/graphics/Banski01/Banski01-008.png

Dependencies among the annotation layers in the Open-Content Text Corpus. Red arrows (image ../../../vol5/graphics/Banski01/Banski01-005.png ) denote the dependencies among the various parts of the hierarchy.

The final example presents the prototype structure of the English subcorpus of the FLEC (Foreign Language Examination Corpus), a learner corpus containing examination essays by students of the University of Warsaw. Its primary aims are to study the transfer of linguistic structures of Polish onto the second language and to measure the inter-rater agreement in order to attain greater objectivity in grading language exams (see [Bański & Gozdawa-Gołębiowski, 2010] for details). The electronic source texts are produced by transcribers (the exams are written in hand), who fill out templates already divided into sentence-sized chunks, and introduce extra markup for unclear passages, special textual features, gaps and the like. The source text is then tokenized according to an agreed tokenization standard (by default, according to whitespace and punctuation, but the English part additionally obeys the CLAWS tokenization rules), and each token is indexed. This becomes the new base that other annotation layers reference, and in effect, the original source text remains only as a backup. This system is close to what [Cummings, 2009] describes; to distinguish it from systems in which the source text receives only light tagging (or no tagging at all, as in the American National Corpus), we use the term "rich-base stand-off system" to refer to it.

Figure 6: Foreign-Language Examination Corpus (FLEC): dependencies in a rich-base stand-off system

image ../../../vol5/graphics/Banski01/Banski01-009.png

Dependencies among the annotation layers in the Foreign-Language Examination Corpus. Red arrows (image ../../../vol5/graphics/Banski01/Banski01-005.png ) denote the dependencies among the various parts of the hierarchy. The black arrow indicates that the segmentation layer takes over all the functions and the content of the source layer, which is retained only for archival purposes but does not participate in further processing of the corpus.

Each essay is rated by at least two instructors, on special forms that make it possible to transcribe the ratings into electronic form. The morphosyntactic layer is added separately, to make it possible to perform searches. In the future, a syntactic level is planned, that will allow for searches based on syntactic criteria for, e.g., specific constructions.

Here is where modularity is required and enforced by practical considerations: the individual parts of the corpus are scheduled to be created at different points in time, as dictated by the availability of the new data, the ability of the transcriber team to cope with the hand-written exams, then to cope with the raters' judgements, while the morphosyntactic descriptions are created.

2.4. Motivation for stand-off representation: summary

This section looked at the motivation for the use of stand-off annotation in linguistic applications. We first looked at the application of the OHCO thesis to linguistic theorizing and found that there was no straightforward relationship between cases where OHCO failed and the preferred encoding strategy. The choice appears to depend on the particular perspective (hinted at rather than defined here with reference to linguistics): whether OHCO-conforming or not, perspectives that reflect the modular nature of the grammar, or that are due to practical issues such as the choice of the particular tagging tool or tagging system, call for dedicated annotation documents. In systems such as the FLEC, the practical issues reach even further and depend on the human annotators of each kind of documents (the essays are planned to be always transcribed as soon as possible, to provide raw material for studies of the lexical content).

Let us reiterate: many of the above issues could be encoded within single files, thanks to the ingenuity of the many designs allowing for non-OHCO representations. But we OWLs are not even going to try them: we want our encoding layers separated, for reasons both theoretical (they encode different pieces of our descriptions and we want to keep it that way) and practical (we want relatively generic systems that "just work" without requiring dedicated tools; if they require too much hassle, we'll just grab a different, existing solution, and not necessarily one based on XML).

3. Stand-off annotation: the semantics of hyperlinks

To our knowledge, the earliest mentions of stand-off annotation, at least in the broad context of the TEI, were made in papers co-authored by Henry Thompson and David McKelvie, with the participation of Amy Isard and Chris Brew ([Thompson & McKelvie, 1997], [McKelvie et al., 1998], [Isard et al., 1998]). They were mostly made in the context of the LT NSL package (later to become LT XML), created at the University of Edinburgh. In these papers, the foundations for stand-off semantics were laid. Below, we look at some of the possible interpretations of stand-off links, the first four defined by the LT NSL group. Much of that has later surfaced in the XInclude and XLink specifications (the latter partially based on TEI pointing techniques).

We can distinguish at least the following kinds of possible interpretations of linking attributes and elements:

  1. inclusion (with or without the loss of metadata, cf. [Thompson & McKelvie, 1997] vs. [Isard et al., 1998]),

    Figure 7: Inclusion semantics

    image ../../../vol5/graphics/Banski01/Banski01-010.png

    Inclusion semantics of hyperlinks as originally presented in [Thompson & McKelvie, 1997], with a simplified notation of the @target attributes. The proposals were unclear about the target metadata (in red) and either involved the loss of it ([Thompson & McKelvie, 1997], [McKelvie et al., 1998]) or preserved it ([Isard et al., 1998]); the latter option is shown in the figure above.

  2. replacement (later turning into XML Inclusions; involving the loss of the pointer metadata),

    Figure 8: Replacement semantics

    image ../../../vol5/graphics/Banski01/Banski01-011.png

    Replacement semantics of hyperlinks as presented in [McKelvie et al., 1998] and [Isard et al., 1998]. This is the straightforward ancestor of XInclude semantics.

  3. inverse replacement ("include everything but the element that I point at, and use me instead of it"); this raises the question of feasibility of implementation if more than one such replacement is performed in a single document;

  4. multiple-point linking (in the future, it became the semantics of the TEI's <link>, specialized into <join> ). See also Listing 4 below.

  5. correspondence semantics, the most underspecified semantic relationship possible (not mentioned in the LT NSL system but logically necessary and somewhat akin to multiple-point linking); correspondence semantics may be enough for visualising applications – i.e., there is no need to derive an extra TEI representation: all that the application has to know is which fragments of one layer correspond to which fragments of another, and that is enough to act on them. Correspondence semantics in also necessary in the case of multimodal corpora, where annotation layers (in)directly address binary streams. In the TEI, there exist a variety of devices for simple pointing, from the @target attribute (and the deprecated @targets), sometimes embedded in the <ptr> or <ref> elements, through the entire range of pointers with added shades of interpretation beyond pointing or linking, such as @corresp, @ref or @ana, among many others (see http://www.tei-c.org/release/doc/tei-p5-doc/en/html/REF-ATTS.html for a complete list of TEI attributes). Note also that the simplest version of multiple-point linking semantics involves this kind of pointing, but at more than one resource at the same time.

  6. merger semantics, possible under limited circumstances, "merge my attributes/content with the attributes/content of the element I am pointing at" – this is a viable possibility for e.g. a morphosyntactic layer composed out of <seg> elements containing feature structures and pointing to a segmentation layer composed of empty <seg> elements, whose only role is to address character spans in the source text. A variation of this scenario with more content is illustrated below.

    Figure 9: Merger semantics

    image ../../../vol5/graphics/Banski01/Banski01-012.png

    Merger semantics, possible if the relevant schemas are non-conflicting (in the extreme case, if both annotation documents are instances of the same schema that also allows the occurrence of both kinds of element content together). The effect is that of unification of descriptions.

  7. reverse inclusion semantics – literal interpretation of the semantics of CES links (see below); untenable for at least practical reasons, though we stress that it has been used mostly in the context of virtual representations, and with such a proviso, reverse inclusion semantics may even be argued to be useful for descriptions of binary streams, which get virtually "adorned" with the annotation information.

    Figure 10: Reverse inclusion (virtual)

    image ../../../vol5/graphics/Banski01/Banski01-013.png

    This type of inclusion can be called, in the context of the system presented here, "reverse inclusion". It represents the literal reading of the most popular characterisation of stand-off markup in the Corpus Encoding Standard documentation. The result is a virtual structure that in fact had to be realised as either straight inclusion or replacement, or else correspondence semantics.

The last kind of semantics is inspired by the prose descriptions of the Corpus Encoding Standard (http://www.cs.vassar.edu/CES/), an SGML-based specialization of the TEI-P3 (cf. [Ide & Véronis, 1993], [Ide, 1998]) and its later XML version, XCES (http://www.cs.vassar.edu/XCES, cf. [Ide et al., 2000]). This standard was an important way to offer OWLs a handy TEI-like tool for quick deployment and proved very popular in corpus-linguistic circles. One of its most important features was that it severely narrowed down the sometimes enormous range of options offered by the unconstrained TEI – this task had already been performed for the OWL, who only needed to choose from among the optional elements and attributes, but crucially, not so much from equivalent ways to annotate texts. Other important features of the XCES were:

  • focus on the implementation of stand-off methodology,

  • three different content models for the three different layers (source text, analysis (morphosyntax and chunks), alignment),

  • re-entrant cesCorpus (supporting the structural encoding of subcorpora),

  • specific recommendations for morphosyntactic and alignment markup.

(X)CES hyperlink semantics was always stated in free prose and typically mentioned “annotations virtually added to the base document” – treated literally, it would result in something like the “reverse inclusion” mentioned above. However, while it is natural to be able to predict and shape the behaviour and composition of the document under the control of the annotator, i.e. the annotation document, it is not necessarily so with respect to the source. This means that at best, we can expect inclusion or replacement semantics here, although what was often meant in the XCES, we believe, might have been simply correspondence semantics, with redundant text fragments copied from the source text layer, somewhat in the manner of multiply-annotated text of e.g. [Goecke et al., 2010], but with no guarantee of exhaustivity of description.

In the listing below, we illustrate some of the above-mentioned concepts of hyperlink semantics with fragments of existing text resources, the National Corpus of Polish (Listing 1) and the Open-Content Text Corpus. The NCP uses correspondence semantics; it is worth pointing out that the word abyś is split into the sentential conjunction aby "in order to" and the person-number clitic ś, which is marked as orthographically adjoined to its host (text tokens are listed in comments above the corresponding <seg> elements).

   <p corresp="text.xml#txt_2-div" xml:id="segm_2-p">
     ...
    <s xml:id="segm_2.40-s">
      ...
      <!-- pragnę -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,207,6)" xml:id="segm_2.33-seg"/>
      <!-- , -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,213,1)" nkjp:nps="true)" 
      xml:id="segm_2.34-seg"/>
      <!-- aby -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,215,3)" xml:id="segm_2.35-seg"/>
      <!-- ś -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,218,1)" nkjp:nps="true)" 
      xml:id="segm_2.36-seg"/>
      <!-- nim -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,220,3)" xml:id="segm_2.37-seg"/>
      <!-- pozostał -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,224,8)" xml:id="segm_2.38-seg"/>
      <!-- ” -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,232,1)" nkjp:nps="true)" 
      xml:id="segm_2.39-seg"/>
      <!-- . -->
      <seg corresp="text_structure.xml#string-range(txt_2.1-ab,233,1)" nkjp:nps="true)" 
      xml:id="segm_2.40-seg"/>
    </s>
    <s xml:id="segm_2.55-s">...</s>
  </p>
    

In the OCTC listings below, we first look at a segmentation file that uses mixed semantics: correspondence semantics for the containing element, <ab> ("anonymous block"), and replacement semantics realised by the XInclude directive using the W3C-defined xpointer() scheme.[12]

<ab l:id="swh_sgm_2-ab" type="para" corresp="text.xml#swh_txt_2-p">
    <seg xml:id="swh_sgm_2.1-seg">
      <xi:include href="text.xml"
        xpointer="xpointer(string-range(id('swh_txt_2-p')/text()[1],'',1,6)[1])"/>
    </seg>
    <seg xml:id="swh_sgm_2.2-seg">
      <xi:include href="text.xml"
        xpointer="xpointer(string-range(id('swh_txt_2-p')/text()[1],'',8,7)[1])"/>
    </seg>
    <seg xml:id="swh_sgm_2.3-seg">
      <xi:include href="text.xml"
        xpointer="xpointer(string-range(id('swh_txt_2-p')/text()[1],'',16,2)[1])"/>
    </seg>
    <seg xml:id="swh_sgm_2.4-seg" rend="glued">
      <xi:include href="text.xml"
        xpointer="xpointer(string-range(id('swh_txt_2-p')/text()[1],'',18,1)[1])"/>
    </seg>
    ...
</ab>
    

The result of resolving XInclude directives – the first segments of the Universal Declaration of Human Rights in Swahili – is provided in Listing 3 below.

 <ab type="para" corresp="text.xml#swh_txt_2-p">
    <seg>Katika</seg>
    <seg>Disemba</seg>
    <seg>10</seg>
    <seg rend="glued">,</seg>
    ...
  </ab>
    

The final listing illustrates one possible take at the multiple-point semantics – a fragment of a document aligning the Polish and the Swahili versions of the Universal Declaration. It does not use the <link> element, usually suggested for this purpose, but rather separate <ptr> elements, for greater granularity (in some cases, many:many relationships between fragments of text must be expressed and using <link> elements with multi-valued @target attributes would be rather tedious).

<div xml:id="pol-swh_aln_2-div" type="tu" part="N" prev="#pol-swh_aln_1.1-linkGrp" org="uniform">
    <linkGrp xml:id="pol-swh_aln_2.1-linkGrp">
      <ptr xml:id="pol-swh_aln_2.1.1-ptr" target="pol/UDHR/text.xml#pol_txt_1-head" type="tuv" 
      xml:lang="pl"/>
      <ptr xml:id="pol-swh_aln_2.1.2-ptr" target="swh/UDHR/text.xml#swh_txt_1-head" type="tuv" 
      xml:lang="sw"/>
    </linkGrp>
    <linkGrp xml:id="pol-swh_aln_2.2-linkGrp">
      <ptr xml:id="pol-swh_aln_2.2.1-ptr" target="pol/UDHR/text.xml#pol_txt_2-p" type="tuv" 
      xml:lang="pl"/>
      <ptr xml:id="pol-swh_aln_2.2.2-ptr" target="swh/UDHR/text.xml#swh_txt_2-p" type="tuv" 
      xml:lang="sw"/>
    </linkGrp>
    ...
</div>
    

In conclusion, it is also advisable to mention the concept of radical stand-off that the XCES evolved into, in the context of the American National Corpus, which keeps the source files in the form of raw UTF-16 text and uses dedicated software (ANCTool) to merge the raw text with the annotations selected by the user, cf. [Ide & Suderman, 2006]. Our interpretation of the the XCES evolving in the context of the ANC and ending up merely as one of the output formats of the ANCTool is that the creators of the ANC have drawn conclusions from the stalled development of the W3C XPointer standard, various versions of which the XCES attempted to use over the years, and finally gave up on it and switched to an in-house tool that made it possible for them to go all the way towards radical stand-off annotation, which undoubtedly has some advantages from the point of view of sustainability of LRs (the texts are kept as read-only, so there is no danger of corrupting them by fixes and adaptations of markup).[13]

4. Stand-off markup in the TEI

We begin by distinguishing two uses of stand-off devices and then concentrate on what features of the XCES can be found implemented in TEI P5.

4.1. local stand-off

Recall that one of the purposes of stand-off annotation is to make it possible to handle overlapping hierarchies and any other sort of conflicting markup. The TEI has several devices for this purpose, mentioned in chapter 20 of the Guidelines. The prototypical example is <join>, aggregating elements that it points to into a virtual object. Other members of the family include <alt>, <span>, and the least semantics-laden <link>. [14]

Due to the fact that pointers in TEI P5 are URI-based, these elements may be used as both "local stand-off" and "remote stand-off" elements (where the former is not an oxymoron and the latter not a tautology): if the metaphor for “stand-off” is paraphrased as “creating/organizing a structure in resource A out of elements of resource B by pointing to them”, then in the cases where the pointing is local, A and B are the same resource. The remaining discussion in this section does not refer to such uses, but we return to them in Section 6. In the remainder of this section, we look at the kind of stand-off annotation that involves pointing across separate documents.

4.2. The converging paths of the XCES and TEI P5

TEI P4 was an XML-ised version of P3, with minimal changes, and only the introduction of TEI P5 saw drastic modifications in the recommendations for linking. To an outside observer, this may be described as the TEI's reabsorption of the modifications introduced by the CES as the latter forked from TEI P3. Apart from important low-level modifications of elements designed to annotate linguistic structure, the important changes were the introduction of a self-nesting <teiCorpus> element (it did not self-nest in P4; this is a feature crucial in resources such as the above-mentioned NCP or OCTC; see also Section 2.3) and the generalization of the concept of stand-off markup, made possible by the switch from IDREF-based pointing to the current URI-based version.[15]

It is worth highlighting an ingenious move in the introduction of stand-off annotation in the TEI, namely the use of the XInclude standard (http://www.w3.org/TR/xinclude/).

The initial version of the XCES (cf. [Ide et al., 2000]) used XLink (then http://www.w3.org/TR/xlink/, currently http://www.w3.org/TR/xlink11/) as the pointing device, with XPointer xpointer() schemas (back then, the schema was called xptr()) as the content of xlink:href. The XPointer xpointer() draft (http://www.w3.org/TR/xptr-xpointer/) was being born right at that moment and nothing forewarned of its remaining at the draft stage for ever, or at least until now. The XLink recommendation was also fresh and promising to see much heavier use than it does today, remaining endemic to only a few specifications. See [Ide, 2000] to glimpse at the optimism that the introduction of new W3C standards brought into the LR community.

TEI P5 documents concerning the use of stand-off annotation (http://www.tei-c.org/Activities/Workgroups/SO/) date from 2003 at the earliest, and at that time, the XInclude recommendation was at least at the Working Draft stage and promising to become at least useful. XInclude explicitly uses replacement semantics (not inclusion semantics, despite the similarity in names) as defined by [Thompson & McKelvie, 1997] (see Section 3), and that must have appeared the perfect solution, the more so that it allowed the TEI to delegate some of the intended functionality to an independent W3C standard, promising to get wide support in XML parsers.[16] Additionally, TEI P5 stand-off implementation was designed with the use of TEI-defined XPointer schemes in mind, which was another brilliant move because nothing (except perhaps for xpointer()'s prolonged draft status, but one should always hope) signalled that these schemes will remain as unimplemented by parsers today as they were at the time of their registration.[17]

5. Problems with implementing TEI stand-off annotation

Stand-off annotation, with all its advantages for language description and documentation, typically requires a dedicated tool to implement the hyperlink semantics, compare e.g. LTXML2 (http://www.ltg.ed.ac.uk/software/ltxml2), ANNIS (http://www.sfb632.uni-potsdam.de/d1/annis/), ANCTool (http://www.americannationalcorpus.org/tools/anctool.html) or applications using MonetDB (http://monetdb.cwi.nl/). In this context, let us repeat that using XInclude accompanied by a set of XPointer schemes was an ingenious move, because in theory, it should allow an OWL to be able to use TEI stand-off with ordinary off-the-shelf tools. In this section, we look at the various factors that conspire to what we believe is the undeserved lack of uptake of TEI stand-off devices. We divide these factors into external and internal, and among the latter, we make a distinction between technological and sociological.

5.1. Technical issues external to the TEI

In this section, much depends on the reader being able to make the distinction between (i) @xpointer as the name of an XInclude attribute that can sometimes contain just a shorthand pointer (an NCname), (ii) XPointer as referring to the entire XPointer Framework, and (iii) xpointer() as referring to one of the XPointer schemes – that is why the sentence “@xpointer can hold XPointer's xpointer()” is meaningful, and true. Unsurprisingly, these terms are notoriously confused on various occasions. Similar remarks concern string-range() as the name of one of the xpointer() scheme's functions, defined by the W3C draft, and string-range() as the name of a TEI-defined XPointer scheme, on a par with xpointer(), element(), and other third-party schemes registered with the W3C. Some of these issues are addressed in a TEI Wiki article at http://wiki.tei-c.org/index.php/XPointer.

As has been mentioned above, the TEI recommendations for stand-off annotation rely on the use of external standards, most importantly XInclude and the XPointer Framework, with its potential for defining third-party XPointer schemes. To cut a long story short: tool support for W3C XPointer schemes other than element() (obligatory for XInclude) and xmlns() does not exist, as far as the popular XML parsers are concerned, and support for third-party schemes is scant.

It might be claimed that these issues are internal to the TEI, after all, because they depend on the TEI's internal choice to use XInclude and to use its own XPointer schemes for pointing into the text. However, there appears to be no alternative to the use of XPointer schemes for pointing into spans of characters (short of unwinding history back to the era of TEI extended pointers whence XPointer comes), and in this sense, the lack of support for XPointer's xpointer() scheme blocks the possible development of support for the TEI-defined schemes, because that support should ideally piggyback (in terms of data structures, basic mechanisms, etc.), on the support for the W3C-defined xpointer().[18]

It has to be noted that there exists a single widely accessible implementation of XInclude that goes beyond the minimum prescribed by the W3C Recommendation: libxml2 (http://xmlsoft.org/) with the xmllint parser that supports limited xpointer() functionality, although unfortunately in a buggy way, so that while it can be tasted and demonstrated, it cannot be employed full-scale.[19]

The lack of tool support made the developers of the National Corpus of Polish resign from using XInclude-based stand-off in favour of the underspecified semantics of the @corresp attribute that simply states correspondence between two elements (or an element and a span of characters), cf. Listing 1. Since this has to be handled by dedicated project tools anyway, it is enough for these tools to read information from @corresp rather than mimic the behaviour of an XInclude processor. This shows the inaccessibility of TEI stand-off to users without technical background or technical support – it does not work out-of-the-box despite the measures that the Guidelines took to ensure that the technique is lucidly described. The OCTC attempts to use W3C-defined xpointer() scheme as much as possible (cf. Listing 2) in order to be able to fall back from W3C technology to TEI schemes when the latter are finally implemented.

[Cayless & Soroka, 2010] point out the possibility of TEI pointers to point outside of their "lawful" domains and out across the whitespace, ignorable or not. This issue is real and actually seen in practice, in the xmllint bugs reported by the present author. Cayless and Soroka's observations should be treated as imposing specific constraints on stand-off pointers, whose ranges, must be located inside the elements identified by each individual pointer; this is trivial in the case of the lower value of the offset (which should not go below 1 in the case of the W3C pointers or below 0 in the case of TEI-defined pointer schemes) but becomes less than trivial when it comes to ensure that the maximal value of the pointer does not extend beyond the addressed string.

5.2. TEI-internal technical issues

We argue here that, despite properly seizing the opportunity for a free ride with W3C specifications, some aspects of the putative reabsorption of XCES innovations into the TEI were not fully addressed.

The TEI diverges from its path of reabsorbing the XCES in that it packages all information, be it the source text or its annotations, into a single teiCorpus/TEI/text format. (Recall that the XCES used three different DTDs for the source text, the morphosyntactic analysis, and the alignment documents; they were different up to the root element.) This is inadequate for two reasons:

  • it strains the semantics of the <text> element (annotations do not contain text, or at least do not have to contain it to be useful),

  • it packages technical annotation documents into the format expected of source text documents, which means that rather than putting a sequence of e.g. <seg> ("segment") elements with morphosyntactic information in feature structures straight into text/body (which is not ideal, as pointed out above, but for many would probably suffice), the developer has to trace the TEI content model and use e.g. the <ab> ("anonymous block") element as a wrapper for <seg>s only for the purpose of satisfying the content model designed for texts; similarly, it is impossible to keep a sequence of <linkGrp> or <spanGrp> elements in text/body – one has to use at least an empty dummy <div> for the document to validate. This feels like a kludge and the developer is tempted at this point to leave the TEI for the XCES or PAULA.

    Note that one way out would be to redefine the content model inside <text> – after all, the TEI offers the mechanism of ODD for this purpose (cf. [Burnard & Rahtz, 2004]). However, that would still mean that

    • annotations are kept under <text>,

    • special effort must be put into designing the ODD beyond a mere selection of the appropriate modules and elements, and

    • the resulting document is not TEI-conformant, because it changes the content model of <text> (cf. chapter 23.3 of the Guidelines). That in itself is no tragedy, but recall that we want to cater, among others, to OWLs (ordinary working linguists), also with a view to the issues raised in the following section. An OWL would often look for an out-of-the-box solution that the XCES promised (although only for morphosyntactic annotation). Furthermore, breaking TEI-conformance may mean that whatever tools there exist for handling the TEI may refuse to handle the non-conformant documents. Again, we are thinking of an OWL who wants it to "just work", and we bear in mind that the recourse to using W3C standards was exactly a step towards ensuring that things "just work". Having an OWL design their own ODD in order to store stand-off annotations does not contribute to that goal.

The users are aware of some of this, cf. a recent TEI-L (http://listserv.brown.edu/archives/cgi-bin/wa?A0=TEI-L) discussion thread (e.g. Martin Holmes's message of Sat, 20 Mar 2010 06:38:00 -0700) on where to keep <linkGrp> and similar elements: no consensus has been reached: there are users who keep it in text/body, text/back, or who would rather keep this in the header (because they feel it is a different kind of data), or, finally, “somewhere else” and we will see presently what this suggestion is.

5.3. Social and sociological issues

Standards are good only insofar they meet the expectations of, and get a chance of getting feedback from, the community that they are targeted at. However, corpus linguists or corpus producers appear to be underrepresented in the TEI community. TEI-L, an otherwise helpful mailing list with a very high signal-to-noise ratio, falls largely silent when corpus-design-related questions are asked, in comparison to e.g. questions regarding the encoding of manuscripts, literary works, or bibliographies. Instead of concluding that TEI-ers with a corpus-design twist are the most unfriendly of TEI-ers, one should rather conclude that there are few such TEI-ers around, and ask why, and whether the XCES has taken them all away.

A corpus linguist, if s/he is going to choose XML (rather than plain text or a RDBMS) as the format of choice, may easily choose the XCES – dated but simple and sufficient for lightly-analysed corpora, or PAULA ([Dipper, 2005]) – not so simple but with an array of technical backup and a bunch of smart people popularizing it, or finally a more dedicated format such as that of the ANC (“extreme stand-off”, cf. [Ide & Suderman, 2006]), a testbed for the nascent ISO Linguistic Annotation Framework ([Ide & Romary, 2007]) and more specifically, for the abstract pivot format, GrAF ([Ide & Suderman, 2007]).

The question is whether the TEI has a chance to become an alternative to these systems for an OWL. Incidentally, when searching the Net for a reference to the origin of the interpretation of “OWL” used here, I came across a similar point made by [Farrar & Moran, 2008]:

“[...] any new approach or technology requires critical mass. If too few in a community use the technology, then it will usually fail. TEI recommendations (using SGML) never caught on with the ordinary working linguist, likely due to the unavailability of tools to produce it. The situation with recent best-practice XML recommendations has been only slightly better.”

— [Farrar & Moran, 2008]

One of the points we want to make in this contribution is that the TEI still has to win some of the corpus linguistic audience in order to kickstart the development-feedback cycle for stand-off corpus encoding. It needs to make a move towards a corpus-oriented OWL, possibly by addressing the issues raised here and by a pressure towards the implementation of a widely-accessible generic tool that supports stand-off architecture. That would be the next of the numerous services to the linguistic and XML community that the TEI has done over the years.

6. A sketch of solutions

The TEI is a very good choice for complex corpus encoding with a view to sustainability and interoperability because, apart from its other virtues, it offers a homogeneous format for encoding all the annotations and storing “formal metadata” in the TEI headers – this point is made in [Bański & Przepiórkowski, 2010], and illustrated in [Przepiórkowski & Bański, 2010]. However, at present, stand-off TEI deployment for such purposes requires dedicated visualisation and query tools acting on the correspondence semantics of hyperlinks (because XInclusions fail due to the lack of support for third-party XPointer schemes anyway), as well as some dedication to simplify parts of the architecture that are unduly complex for someone who wants to “just do it”, just create a stand-off annotation document without having to create a mock text document in the process.

Some of the necessary pieces of the puzzle are already there. Recall that the (X)CES used three different content models for annotating the source text, the annotations, and the alignment. We do not want to argue for that – that would not be as homogeneous a format as what the TEI offers currently. Instead, we would like to point out that it is possible to reabsorb the (X)CES inventions more fully into the current TEI model, by keeping text under <text>, and non-text elsewhere. Let us have a look at the content model of the <TEI> element:

element TEI
{
   att.global.attributes,
   attribute version { xsd:decimal }?,
   ( teiHeader, ( ( model.resourceLike+, text? ) | text ) )
}
    

model.resourceLike contains <facsimile> (for digital facsimiles) and <fsdDecl> (for feature system declarations). Another element that is planned to be included in this model, defined by a planned new chapter of the Guidelines devoted to genetic editions, is <document> (the physical object, the manuscript or other primary source, comprising one or more written surfaces, see http://www.tei-c.org/SIG/Manuscripts/genetic.html). Another addition, suggested by [Boot, 2009], is <dataSection> (to store, e.g., <linkGrp> elements that just do not fit under <text>). In two e-mails to the TEI-L (on 21 Mar 2010 and on 22 Mar 2010), the present author has suggested the introduction of <standOff> for the same purpose. What this shows it that a need for an extra sibling of <text> is recognized in the community, and, in the case of <document>, it is actually being implemented. Implementing an element that could store stand-off markup would be the final, and in our view necessary, step in reabsorbing the XCES innovations into the TEI. It would simplify the content model for the annotation creators, making it possible for them not to abuse the semantics of <text>, and at the same time not to be bound by the requirements of <text>'s content model.

We sketch the possible configurations resulting from implementing this solution using <standOff> because the name of this element corresponds with the topic of the present contribution. We note, however, that the name "standOff" is only used for the same of discussion and that e.g. Peter Boot's suggestion for the name of the element in question, namely <dataSection> sounds just as good, although perhaps too generic (everything here is data).

Recall that the TEI contains a range of elements with uses that we have referred to in Section 4.1by the mildly fortunate term "local stand-off". They are elements that may sometimes be perfectly justified inside <text>, but there have been suggestions to move them into the teiHeader or (as in [Boot, 2009]) to move them to a sibling of <text>. We suggest that two of these solutions can be used, depending on the annotating task at hand (we reject the option of locating these elements in the header). Those who would rather separate the "local stand-off" elements from text proper, could use three child elements of <TEI> at the same time: { teiHeader, standOff, text }. The third group, the creators of the "classical" stand-off annotation levels, would use { teiHeader, text } for text documents but { teiHeader, standOff} for annotations. This is not the place to suggest the content model of the putative <standOff> element – suffice it to say that we would expect it to hold, among others, elements from the "analysis" and "linking" parts of the TEI module inventory.[20]

It is also worth mentioning that, for the purpose of establishing character offsets in XPointer schemes, the W3C proposals point to character segments and use "1" as the initial offset, the TEI and LAF proposals look at inter-character points and use "0" as the initial offset. We consider this an unfortunate difference as it is not user-friendly and the cost of adapting to W3C model, while the TEI schemes are not implemented yet, should be minimal.

Lastly, community pressure (or simply funding) is needed to implement XPointer extensions in a generic XML tool (the best candidate being libxml2 by Daniel Veillard, with the xmllint parser, because it already has some of this functionality that no other popular and freely available parser has), so that the ingenious TEI stand-off system based on XML Inclusions can do something more than merely look nice.[21]

Our claim is that it should be possible for a corpus-oriented OWL with the basic TEI awareness to read portions of chapter 15 on language corpora and chapter 16 on stand-off linking and chapter 17 on analytical mechanisms in order to be able to construct a simple working stand-off-annotated corpus prototype that they will be able to visualise and perhaps even to query. Ideally, even that step should be simplified and expressed as a single chapter-like set of recommendations (possibly in the form a TEI ODD file) targeted specifically at corpus linguists.[22]

7. Conclusion

The motivation for the present analysis was to flesh out certain inadequacies of the TEI approach to stand-off annotation, in order to see how many of the problems have causes internal to the TEI Guidelines, and how many can be attributed to external factors, such as the lack of sufficiently developed generic XML tools or the inadequacies of standards assumed by the TEI. The ultimate question, therefore, is: should the TEI be used for modern stand-off text encoding at all, or should developers turn to other formats, such as the excellent PAULA toolkit (cf. [Dipper, 2005]), the slightly aged XCES ([Ide et al., 2000]) or the more generic but still incomplete LAF family of standards ([Ide & Romary, 2007]).

If the TEI wants to become a viable alternative to other formats, it should ensure that an OWL can easily implement and use a prototype stand-off corpus. This is conditioned by two factors: one internal (making content models of stand-off documents maximally friendly and packaging them for out-of-the-box deployment) and one external (the lack of generic parsing tools that would implement XInclude with third-party XPointers schemes, ideally as modules). Both issues are solvable. Both would normally be solved by community pressure but the community (sub-community of NLP-oriented TEI users) has yet to be formed and in its absence, it is the rest that should act, for the sake of making the TEI community richer and more dynamic, and in this way, to supply all of us with new ideas and research topics, and new tools (including better support for stand-off document creation, visualisation and querying) to go with them, because tools follow users. We have identified three classes of problems, all tied together. In order to move on, the tie should be cut, preferably at a few places at the same time.

[Wittern et al., 2009] mention that a separate chapter concerning corpus annotation has been considered for inclusion in the Guidelines but never ended up finished and included. We believe to have shown here that that chapter, after considerable revisions, might be one of the ways in which the TEI reaches out to OWLs interested in corpora, whether of the purely textual or the multimodal kind. The nascent Special Interest Group for linguists (http://wiki.tei-c.org/index.php/TEI_for_linguists) is another step towards that goal.

Acknowledgements

I wish to thank the six anonymous Balisage reviewers for their encouraging and helpful comments. It was not possible to implement all of the suggestions in a single article, but I did my best. The responsibility for the remaining errors naturally remains my own.

I would like to express my gratitude to B. Tommie Usdin for her enormous patience and support, without which I would not be able to complete this article in time for publication.

Bibliography

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[1] TEI stands for Text Encoding Initiative – the project, the organization and the community whose primary deliverable are the Guidelines for Electronic Text Encoding and Interchange ([TEI Consortium, 2010]), summing up or recommending best practices for the encoding of a multitude of varieties of textual phenomena (see [Jannidis, 2009] for a concise description and [Renear, 2004] for broader discussion from the perspective of version P4 and for the placement of the TEI in the context of text encoding studies). As [Renear, 2004] puts it, after HTML, the TEI is probably the most extensively used SGML/XML text encoding system in academic applications. It is also worth pointing out that the TEI partially informed the development of the XLink and XPointer W3C recommendations, and also the ISO Feature Structure Representation standard (ISO 24610-1). Currently, there is some interaction between the TEI encoding methods and the emerging Language Annotation Framework (LAF) standards, created by the ISO TC 37 SC 4 committee.

The TEI began in 1987 and has been through several versions, coming all the way from SGML to XML and following closely the developments in the field of XML specifications. As we shall show below, some of the attempts are still to be completed. The current version of the TEI is TEI P5 – see [Wittern et al., 2009] for a brief account of the innovations that this version introduces, and [Cummings, 2008] for a broader view in the context of the Humanities.

[2] The acronym OWL in the sense of "ordinary working linguist" predates the Web Ontology Language by a few decades. A relevant book reference is e.g. [Lawler & Aristar Dry, 1998], but as Michael Maxwell (personal communication) tells me, the term was in use among field linguists at least in the 80's. He goes on to say that its coining on the part of field linguists (particularly SIL field linguists) was a reaction to the disdain with which some theoretical linguists looked down on field work, or at least on field work that wasn't grounded in some (acceptable) theory..

[3] See [DeRose et al., 1990] for a manifesto, [Renear et al., 1993] for a re-evaluation, and both [Renear, 2004] and [Cummings, 2008] for discussion of OHCO in the context of the TEI.

[4] Morphophonology, defined roughly as dealing with the alternation of phonemes (the abstract contrastive elements of speech), was often – and with some embarrassment – kept in dark corners of most structuralist theories of at least the first half of the 20th century.

[5] [doesn]['t] and the lemmatized (or string-mapped) [does][not] are also viable strategies; see [Ide & Suderman, 2007] and [Chiarcos et al., 2009] for more examples and discussion; see also the tokenization section of the ACL Special Interest Group for Annotation wiki (http://cims.nyu.edu/~meyers/SIGANN-wiki/wiki/index.php/Tokenization_Standards) for a concise and up-to-date summary of the various issues concerning splitting the textual stream into interesting units.

[6] See [Chiarcos et al., 2009] for discussion of cases where such fundamentally different tokenizations need to be merged and for a proposal of a merging algorithm.

[7] Although some correlations between the violations of [Renear et al., 1993]'s OHCO-3 and the placement of the offending structures within the same document suggest themselves, I believe this issue – if it is a valid issue at all – to be beyond the scope of the present submission.

[8] Naturally, stand-off annotation is not restricted to XML applications alone, but this is what we take as our focus here.

[9] It is also worth mentioning that in some cases, the objects of interest do not form a contiguous whole and neither is there any hierarchy to talk about. For example, the layer of word sense disambiguation need only contain references to the particular forms of the lexemes disambiguated in the accompanying lexicon. This is exactly the case in the National Corpus of Polish, cf. Figure 4.

[10] On sustainability of LRs in general, see e.g. [Bird and Simons, 2003] and [Simons & Bird, 2008]. On sustainability in the context of the TEI, see [Witt et al., 2009b].

For more on interoperability of LRs, see e.g. [Ide & Romary, 2007] or [Witt et al., 2009a]. We use the stand-off terminology in accordance with [Goecke et al., 2010].

[11] In the earlier version of the corpus, for technical reasons, this layer was based on the morphosyntactic disambiguation layer (2.), which is reflected in some of the early publications.

[12] The NCP listing is taken from http://nlp.ipipan.waw.pl/TEI4NKJP/ and slightly modified. The OCTC examples in Listing 2 and Listing 4 are taken from the OCTC SVN repository. They use W3C-defined pointers, which can be used as a fallback from the TEI XPointer version (not shown in the listing).

[13] Another way to cope with the lack of XPointer support has been to use XPointer-like attributes with in-house tools. This is the case of the PAULA, as illustrated by a fragment of Figure 1 in [Dipper, 2005]:

<mark id="tok 1" xlink:href="#xpointer(string-range(//body,'',1,3)))"/>
Such XPointers, however, would under the W3C definition return sequences of spans rather than single spans -- what is missing is a position predicate that can be found in the OCTC listing above. This is not meant as criticism of the PAULA approach but merely as an observation that XPointer xpointer() syntax, due to the lack of tools implementing it, started a life of its own, being used as a concise replacement for what could be, e.g. @node, @from, and @length attributes.

[14] We ignore <ptr> and <ref>, because these are, respectively, the plain and the adorned instances of the infrastructure that makes <join> and its kin usable. Likewise, we ignore the role of pointing attributes on elements other than those mentioned here.

[15] This is not to claim that the change was unidirectional and conditioned by the (X)CES. In fact, as Lou Burnard (personal communication) points out, TEI P3 already had a basis for stand-off systems thanks to the <xptr> element that handled external pointing. This has of course got streamlined in P5, with the adoption of uniform pointing devices. For more on these changes, cf. [Wittern et al., 2009]. Our usage of the term "reabsorption" concerns the fact that the CES/XCES was/is a specialized system in which, by its very nature and due to the theoretical assumptions that shaped it, language-resource-oriented solutions had to be introduced from the outset. Whether their emergence in TEI P4 and P5 was only a matter of convergence of two independent lines of thought or whether the more specialized standard informed the more general one is something that I do not concern myself with, although I would welcome a situation whereby one open project benefits from the findings of another, rather than waste its time on reinventing the wheel.

[16] This prediction was eventually borne out, after some issues of infoset merger were solved, especially those concerning @xml:base fixup (cf. http://norman.walsh.name/2005/04/01/xinclude); nowadays, full XInclude support is among the basic features that all general-purpose parsers are expected to have.

[17] For an explanation of the various XPointer-related terms and some guidance across the labyrinth of terminology, see http://wiki.tei-c.org/index.php/XPointer. The W3C repository for third-party XPointer schemes is located at http://www.w3.org/2005/04/xpointer-schemes/.

[18] It may be speculated that, since XInclude already allows for including raw text (as a whole resource), it should also be allowed to address into raw text (in the extreme stand-off fashion), with appropriate XPointer schemes. For example, an appropriate scheme for addressing raw text could be a variant of the string-range() function, e.g.

              (string*)text-range([string-to-match], offset, length)
              (string)text-range(offset, length)
              (string)text-span(startoffset, endoffset)
            
Several issues would have to be addressed at the application level (e.g. skipping the BOM character, recognizing character encoding, etc.), but on the whole, it does not feel particularly exotic and would only require lifting the XInclude ban on the simultaneous presence of @parse=”text” and @xpointer as well as an adjustment in the MIME types enumerated in the XPointer Framework Recommendation. Both issues are demand-driven and potentially interrelated: if XInclude were able to handle addressing into raw text, interest in developing modular implementations of XPointer schemes might follow. This is further elaborated on in [Bański, 2010].

[19] The two outstanding bugs that block the use of W3C-defined schemes in xmllint are reported in the Gnome Bugzilla, at https://bugzilla.gnome.org/show_bug.cgi?id=620190 and https://bugzilla.gnome.org/show_bug.cgi?id=620195. The OCTC now implements workarounds for both these bugs.

[20] The NCP and to a slightly larger extent the OCTC would introduce one more innovation in the handling of annotation structures of the same text: complexes involving a { teiHeader, text } and multiple instances of { teiHeader, standOff} would become, abstractly, <teiHeader, { text, standOff(1..N)}> (where angle brackets denote an ordered pair, and curly brackets a set), because in both corpora, the header is shared (XIncluded) among all the relevant documents. We leave the interesting consequences of such a setup for another occasion.

[21] Note that the fact that XInclude can act on IDs is enough only for the purpose of including pieces of an XML tree (or entire trees). However, stand-off annotation is (or can be, with the replacement semantics of XInclude rather than the correspondence semantics of @corresp), not about including elements and thus merging infosets – given the need for different content models for different annotation layers, stand-off annotation is about including text. A little step on the way towards this goal is offered by the xpath1() XPointer extension, supported by some parsers. Being an XPath implementation, however, it cannot support addressing into the text content, which misses a crucial part of the entire enterprise (one cannot XInclude substrings for the purpose of defining a segmentation level, with segments often defined over substrings of orthographic words, cf. [Bański & Przepiórkowski, 2009]).

[22] It is worth mentioning two TEI-stand-off-oriented tools refined in the National Corpus of Polish and available under the GPL: poliquarp, for querying and concordancing multi-level TEI corpora and anotatornia, for manual annotation of individual layers.

Author's keywords for this paper: TEI; stand-off annotation; hyperlink semantics; corpus encoding

Piotr Bański

Assistant Professor

Institute of English Studies, University of Warsaw

Piotr Bański is an Assistant Professor at the Institute of English Studies, University of Warsaw, where he teaches formal linguistics (primarily linguistic morphology and syntax), lexicography, and the history of English. He has participated, in the role of the XML architect, in projects building the IPI PAN corpus of Polish (encoded in the XCES) and the National Corpus of Polish (a 109-word resource encoded in multi-level stand-off TEI). He is co-administrator of two TEI-based multilingual projects, FreeDict (grouping bilingual dictionaries) and Open-Content Text Corpus (with multiple monolingual and aligned parts; currently at the alpha stage).