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9 <head>
10 <title>Vocabularies in the Virtual Observatory</title>
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13 <meta name="DC.subject" content="IVOA, Virtual Observatory, Vocabulary"/>
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41 <body>
42 <div class="head">
43 <table>
44 <tr><td><a href="http://www.ivoa.net/"><img alt="IVOA logo" src="http://ivoa.net/icons/ivoa_logo_small.jpg" border="0"/></a></td></tr>
45 </table>
46
47 <h1>Vocabularies in the Virtual Observatory, v@VERSION@</h1>
48 <h2>IVOA Working Draft, @RELEASEDATE@ [DRAFT $Revision$]</h2>
49 <!-- $Revision$ $Date$ -->
50
51 <dl>
52 <dt>Working Group</dt>
53 <dd><em><a href="http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics">Semantics</a></em></dd>
54
55 <dt>This version</dt>
56 <dd><span class='url'>http://www.ivoa.net/twiki/bin/view/IVOA/IvoaSemantics</span><br/>
57 <span class='url'>@BASEURI@.xhtml</span></dd>
58
59 <dt>Latest version</dt>
60 <dd><span class='url'>@DISTURI@</span></dd>
61
62 <dt>Editors</dt>
63 <dd>Alasdair J G Gray,
64 <a href='http://nxg.me.uk/norman#norman' >Norman Gray</a>,
65 Frederic V Hessman and
66 Andrea Preite Martinez</dd>
67
68 <dt>Authors</dt>
69 <dd>
70 <span property="dc:creator">Sébastien Derriere</span>,
71 <span property="dc:creator">Alasdair J G Gray</span>,
72 <span property="dc:creator">Norman Gray</span>,
73 <span property="dc:creator">Frederic V Hessman</span>,
74 <span property="dc:creator">Tony Linde</span>,
75 <span property="dc:creator">Andrea Preite Martinez</span>,
76 <span property="dc:creator">Rob Seaman</span> and
77 <span property="dc:creator">Brian Thomas</span>
78 </dd>
79 </dl>
80 <hr/>
81 </div>
82
83 <div class="section-nonum" id="abstract">
84 <p class="title">Abstract</p>
85
86 <div class="abstract">
87 <p>As the astronomical information processed within the <em>Virtual Observatory
88 </em> becomes more complex, there is an increasing need for a more
89 formal means of identifying quantities, concepts, and processes not
90 confined to things easily placed in a FITS image, or expressed in a
91 catalogue or a table. We proposed that the IVOA adopt a standard
92 format for vocabularies based on the W3C's <em>Resource Description
93 Framework</em> (RDF) and <em>Simple Knowledge Organisation System</em>
94 (SKOS). By adopting a standard and simple format, the IVOA will
95 permit different groups to create and maintain their own specialised
96 vocabularies while letting the rest of the astronomical community
97 access, use, and combined them. The use of current, open standards
98 ensures that VO applications will be able to tap into resources of the
99 growing semantic web. Several examples of useful astronomical
100 vocabularies are provided, including work on a common IVOA thesaurus
101 intended to provide a semantic common base for VO applications.</p>
102 </div>
103
104 </div>
105
106 <div class="section-nonum" id="status">
107 <p class="title">Status of this document</p>
108
109 <p>This is (<strong>an internal draft of</strong>) an IVOA Working
110 Draft. The first release of this document was
111 <span property="dc:date">@RELEASEDATE@</span>.</p>
112
113 <p>This document is an IVOA Working Draft for review by IVOA members
114 and other interested parties. It is a draft document and may be
115 updated, replaced, or obsoleted by other documents at any time. It is
116 inappropriate to use IVOA Working Drafts as reference materials or to
117 cite them as other than <q>work in progress</q>.</p>
118
119 <p>A list of current IVOA Recommendations and other technical
120 documents can be found at
121 <span class='url' >http://www.ivoa.net/Documents/</span>.</p>
122
123 <h3>Acknowledgments</h3>
124
125 <p>We would like to thank the members of the IVOA semantic working
126 group for many interesting ideas and fruitful discussions.</p>
127 </div>
128
129 <h2><a id="contents" name="contents">Table of Contents</a></h2>
130 <?toc?>
131
132 <hr/>
133
134 <div class="section" id="introduction">
135 <p class="title">Introduction</p>
136
137 <div class="section">
138 <p class="title">Vocabularies in astronomy</p>
139
140 <p>Astronomical information of relevance to the Virtual Observatory
141 (VO) is not confined to quantities easily expressed in a catalogue or
142 a table.
143 Fairly simple things such as position on the sky, brightness in some
144 units, times measured in some frame, redshifts, classifications or
145 other similar quantities are easily manipulated and stored in VOTables
146 and can currently be identified using IVOA Unified Content Descriptors
147 (UCDs) <span class="cite">std:ucd</span>.
148 However, astrophysical concepts and quantities use a wide variety of
149 names, identifications, classifications and associations, most of
150 which cannot be described or labelled via UCDs.</p>
151
152 <p>There are a number of basic forms of organised semantic knowledge
153 of potential use to the VO, ranging from informal <q>folksonomies</q>
154 (where users are free to choose their own labels) at one extreme, to
155 formally structured <q>vocabularies</q> (where the label is drawn from
156 a predefined set of definitions, and which can include relationships between
157 labels) and <q>ontologies</q> (where the domain is captured in a
158 formal data model) at the other.
159 More formal definitions are presented later in this document.
160 </p>
161
162 <p>An astronomical ontology is necessary if we are to have a computer
163 (appear to) `understand' something of the domain.
164 There has been some progress towards creating an ontology of
165 astronomical object types <span
166 class="cite">std:ivoa-astro-onto</span> to meet this need.
167 However there are distinct use cases for letting human users find
168 resources of interest through search and navigation of the information space.
169 The most appropriate technology to meet these use cases derives from
170 the Information Science community, that of <em>controlled
171 vocabularies, taxonomies and thesauri</em>.
172 In the present document, we do not distinguish between controlled
173 vocabularies, taxonomies and thesauri, and use the term
174 <em>vocabulary</em> to represent all three.
175 </p>
176
177 <p>One of the best examples of the need for a simple vocabulary within
178 the VO is VOEvent <span class="cite">std:voevent</span>, the VO
179 standard for supporting rapid notification of astronomical events.
180 This standard requires some formalised indication of what a published
181 event is `about', in a formalism which can be used straightforwardly
182 by the developer of relevant services. See <span class='xref'
183 >usecases</span> for further discussion.</p>
184
185 <p>A number of astronomical vocabularies have been created, with a
186 variety of goals and intended uses. Some examples are detailed below. </p>
187
188 <ul>
189
190 <li>The <em>Second Reference Dictionary of the Nomenclature of
191 Celestial Objects</em> <span class="cite">lortet94</span>, <span
192 class="cite">lortet94a</span> contains 500 paper pages of astronomical
193 nomenclature</li>
194
195 <li>For decades professional journals have used a set of reasonably
196 compatible keywords to help classify the content of whole articles.
197 These keywords have been analysed by Preite Martinez &amp; Lesteven
198 <span class="cite">preitemartinez07</span>, who derived a
199 set of common keywords constituting one of the potential bases for a
200 fuller VO vocabulary. The same authors also attempted to derive a set
201 of common concepts by analysing the contents of abstracts in journal
202 articles, which should comprise a list of tokens/concepts more
203 up-to-date than the old list of journal keywords. A similar but less
204 formal attempt was made by Hessman <span class='cite'>hessman05</span>
205 for the VOEvent working group, resulting in a similar list <span
206 class="todo">[TODO] Check differences from the A&amp;A
207 list</span>.</li>
208
209 <li>Astronomical databases generally use simple sets of keywords
210 – sometimes hierarchically organised – to help users make queries.
211 Two examples from very
212 different contexts are the list of object types used in the <a
213 href="http://simbad.u-strasbg.fr">Simbad</a> database and the search
214 keywords used in the educational Hands-On Universe image database
215 portal.</li>
216
217 <li>The Astronomical Outreach Imagery (AOI) working group has created
218 a simple taxonomy for helping to classify images used for educational
219 or public relations <span class="cite">std:aoim</span>. See section
220 <span class='xref'>vocab-aoim</span>.</li>
221
222 <!--
223 <li>The Hands-On Universe project (see <span class='url'
224 >http://sunra.lbl.gov/telescope2/index.html</span>) has maintained a
225 public database of images for use by the general public since the
226 1990s. The images are very heterogeneous, since they are gathered from
227 a variety of professional, semi-professional, amateur, and school
228 observatories, so a simple taxonomy is used to facilitate browsing
229 by the users of the database.</li>
230 -->
231
232 <li>In 1993, Shobbrook and Shobbrook published an Astronomy Thesaurus
233 endorsed by the IAU <span class='cite' >shobbrook92</span>. This
234 collection of nearly 3000 terms, in five languages, is a valuable
235 resource, but has seen little use in recent years. Its very size,
236 which gives it expressive power, is a disadvantage to the extent that
237 it is consequently hard to use. See <span class='xref'>vocab-iau93</span>.</li>
238
239 <li>The VO's Unified Content Descriptors <span class='cite'
240 >std:ucd</span> (UCD) constitute the main controlled vocabulary of the
241 IVOA and contain some taxonomic information. However, UCD has some
242 features which supports its goals, but which make it difficult to use
243 beyond the present applications of labelling VOTables: firstly, there
244 is no standard means of identifying and processing the contents of the
245 text-based reference document; secondly, the content cannot be openly
246 extended beyond that set by a formal IVOA committee without going
247 through a laborious and time-consuming negotiation process of
248 extending the primary vocabulary itself; and thirdly, the UCD
249 vocabulary is primarily concerned with data types and their
250 processing, and only peripherally with astronomical objects (for
251 example, it defines formal labels for RA, flux, and bandpass, but does
252 not mention the Sun). See <span class='xref'>vocab-ucd1</span>.</li>
253
254 </ul>
255 </div>
256
257 <div class='section' id='usecases'>
258 <p class='title'>Use-cases, and the motivation for formalised vocabularies</p>
259
260 <p>The most immediate high-level motivation for this work is the
261 requirement of the VOEvent standard <span class='cite'
262 >std:voevent</span> for a controlled vocabulary usable in the
263 VOEvent's <code>&lt;why/&gt;</code> and <code>&lt;what/&gt;</code>
264 elements, which describe what
265 sort of object the VOEvent packet is describing, in some broadly
266 intelligible way. For example a `burst' might be a gamma-ray burst
267 due to the collapse of a star in a distant galaxy, a solar flare, or
268 the brightening of a stellar or AGN accretion disk, and having an
269 explicit list of vocabulary terms can help guide the event publisher
270 into using a term which will be usefully precise for the event's
271 consumers. A free-text label can help here (which brings us into the
272 area sometimes referred to as `folksonomies'), but the astronomical
273 community, with a culture sympathetic to international agreement, can
274 do better.</p>
275
276 <p>The purpose of this proposal is to establish a set of conventions for
277 the creation, publication, use, and manipulation of
278 astronomical vocabularies within the Virtual Observatory, based upon
279 the W3C's SKOS standard. We include as appendices to this proposal
280 formalised versions of a number of existing vocabularies, encoded as
281 SKOS vocabularies <span class="cite">std:skoscore</span>.</p>
282
283 <p>Specific use-cases include the following.</p>
284 <ul>
285 <li>A user wishes to process all events concerning supernovae, which
286 means that an event concerning a type 1a supernova must be understood to be
287 relevant. [This supports a system working autonomously, filtering
288 incoming information]</li>
289
290 <li>A user is searching an archive of VOEvents for microlensing
291 events, and retrieves a large number of them; the search interface may
292 then prompt her to narrow her search using one of a set of terms
293 including, say, binary lens events. [This supports so-called `semantic
294 search', providing semantic support to an interface which is in turn
295 supporting a user]</li>
296
297 <li>A user wishes to search for resources based on the
298 journal-supported keywords in a paper; they might either initiate this by
299 hand, or have this done on their behalf by a tool which can extract
300 the keywords from a PDF. The keywords are in the A&amp;A vocabulary,
301 and mappings have been defined between this vocabulary and others,
302 which means that the query keywords is translated automatically
303 into those appropriate for a search of an outreach image database
304 (everyone likes pretty pictures), the VO Registry, a set of Simbad
305 object types, and one or more concepts in more formal ontologies. The
306 search interface is then able to support the user browsing up and down
307 the AOIM vocabulary, and a specialised Simbad tool is able to take
308 over the search, now it has an appropriate starting place. [This
309 supports interoperability, building on the investments which
310 institutions and users have made in existing vocabularies]</li>
311
312 </ul>
313
314 <p>It is not a goal of this standard, as it is not a goal of SKOS, to
315 produce knowledge-engineering artefacts which can support elaborate
316 machine reasoning – such artefacts would be very valuable, but require
317 much more expensive work on ontologies. As the supernova use-case
318 above illustrates, even simple vocabularies can support useful machine
319 reasoning.</p>
320
321 <p>It is also not a goal of this standard to produce new vocabularies, or
322 substantially alter existing ones; instead, the vocabularies included
323 below in <span class='xref'>distvocab</span> are directly derived from
324 existing vocabularies, with adjustments to make them structurally
325 compatible with SKOS, or to remove (in the case of the IAU-93 and
326 IVOAT pair) significant anachronisms. It therefore follows that the
327 ambiguities, redundancies and incompleteness of the source vocabularies
328 are faithfully represented in the distributed SKOS vocabularies.</p>
329
330 <p>The reason for both of these limitations is that vocabularies are
331 extremely expensive to produce, maintain and deploy, and we must
332 therefore rely on such vocabularies as have been developed, and
333 attached as metadata to resources, by others. Such vocabularies are
334 less rich or less coherent than we might prefer, but widely enough
335 deployed to be useful.</p>
336
337 </div>
338
339 <div class="section">
340 <p class="title">Formalising and managing multiple vocabularies</p>
341
342 <p>We find ourselves in the situation where there are multiple
343 vocabularies in use, describing a broad range of resources of interest
344 to professional and amateur astronomers, and members of the public.
345 These different vocabularies use different terms and different
346 relationships to support the different constituencies they cater for.
347 For example, <q>delta Sct</q> and <q>RR Lyr</q> are terms one would
348 find in a vocabulary aimed at professional astronomers, associated
349 with the notion of <q>variable star</q>; however one would
350 <em>not</em> find such technical terms in a vocabulary intended to
351 support outreach activities.</p>
352
353 <p>One approach to this problem is to create a single consensus
354 vocabulary, which draws terms from the various existing vocabularies
355 to create a new vocabulary which is able to express anything its users
356 might desire. The problem with this is that such an effort would be
357 very expensive, both in terms of time and effort on the part of those
358 creating it, and to the potential users, who have to learn
359 to navigate around it, recognise the new terms, and who have to be
360 supported in using the new terms correctly (or, more often,
361 incorrectly).</p>
362
363 <p>The alternative approach to the problem is to evade it, and this is
364 the approach taken in this document. Rather than deprecating the
365 existence of multiple overlapping vocabularies, we embrace it,
366 help interest groups formalise as many of them as are appropriate, and
367 standardise the process of formally declaring the relationships between
368 them. This means that:</p>
369 <ul>
370 <li>The various vocabularies are allowed to evolve separately, on
371 their own timescales, managed either by the IVOA, individual working
372 groups within the IVOA, or by third parties;</li>
373
374 <li>Specialised vocabularies can be developed and maintained by the
375 community with the most knowledge about a specific topic, ensuring
376 that the vocabulary will have the most appropriate breadth, depth, and
377 precision;</li>
378
379 <li>Users can choose the vocabulary or combination of vocabularies most
380 appropriate to their situation, either when annotating resources, or
381 when querying them; and</li>
382
383 <li>We can retain the previous investments made in vocabularies by
384 users and resource owners.</li>
385
386 </ul>
387
388
389 </div>
390
391 </div>
392
393 <div class='section'>
394 <p class='title'>SKOS-based vocabularies (informative)</p>
395
396 <p>In this section, we introduce the concepts of SKOS-based
397 vocabularies, and the technology of mapping between them. We describe
398 some additional requirements for IVOA vocabularies in the next
399 section, <span class='xref' >publishing</span>.</p>
400
401 <div class="section" id='vocab'>
402 <p class="title">Selection of the vocabulary format</p>
403
404 <p>After extensive online and face-to-face discussions, the authors have
405 brokered a consensus within the IVOA community that
406 formalised vocabularies should be published at least in SKOS (Simple Knowledge
407 Organisation System) format, a W3C draft standard application of RDF to the
408 field of knowledge organisation <span
409 class="cite">std:skoscore</span>. SKOS draws on long experience
410 within the Library and Information Science community, to address a
411 well-defined set of problems to do with the indexing and retrieval of
412 information and resources; as such, it is a close match to the problem
413 this document is addressing.</p>
414
415 <p>ISO 5964 <span class='cite' >std:iso5964</span> defines a number of
416 the relevant terms (ISO 5964:1985=BS 6723:1985; see also <span
417 class='cite' >std:bs8723-1</span> and <span class='cite'
418 >std:z39.19</span>), and some of the (lightweight) theoretical
419 background. The only technical distinction relevant to this document
420 is that between `vocabulary' and `thesaurus': BS-8723-1 defines a
421 thesaurus as a</p>
422 <blockquote>
423 Controlled vocabulary in which concepts are represented by preferred
424 terms, formally organized so that paradigmatic relationships between
425 the concepts are made explicit, and the preferred terms are
426 accompanied by lead-in entries for synonyms or quasi-synonyms.
427 <!-- NOTE:
428 The purpose of a thesaurus is to guide both the indexer and the
429 searcher to select the same preferred term or combination of preferred
430 terms to represent a given subject. -->
431 (BS-8723-1, sect. 2.39)
432 </blockquote>
433 <p>with a similar definition in ISO-5964 sect. 3.16. The paradigmatic
434 relationships in question are those relating a term to a <q>broader</q>,
435 <q>narrower</q> or more generically <q>related</q> term. These
436 notions have an operational definition: any resource
437 retrieved as a result of a search on a given term will also be
438 retrievable through a search on that term's <q>broader term</q>
439 (<q>narrower</q> is a simple inverse, so that for any pair of terms,
440 if <code>A skos:broader B</code>, then <code>B skos:narrower A</code>;
441 a term may have multiple narrower and broader terms).
442 This is not a subsumption relationship, as there is no implication
443 that the concept referred to by a narrower term is of the same
444 <em>type</em> as a broader term.</p>
445
446 <p>Thus <strong>a vocabulary (SKOS or otherwise) is not an
447 ontology</strong>. It has lighter and looser semantics than an
448 ontology, and is specialised for the restricted case of resource
449 retrieval. Those interested in ontological analyses can easily
450 transfer the vocabulary relationship information from SKOS to a formal
451 ontological format such as OWL <span class='cite' >std:owl</span>.</p>
452
453 <p>The purpose of a thesaurus is to help users find resources they
454 might be interested in, be they library books, image archives, or VOEvent
455 packets.</p>
456
457 </div>
458
459 <div class='section'>
460 <p class='title'>Content and format of a SKOS vocabulary</p>
461
462 <p>A published vocabulary in SKOS format consists of a set of
463 <q>concepts</q> – an example concept capturing the
464 vocabulary information about spiral galaxies is provided in the <a
465 href='#figexample' >Figure below</a>, with the RDF shown in both
466 RDF/XML <span class='cite' >std:rdfxml</span> and Turtle notation <span
467 class='cite' >std:turtle</span> (Turtle is similar to the more
468 informal N3 notation). The elements of a concept are detailed
469 below.</p>
470
471 <center>
472 <p><a name='figexample' >Figure: examples of SKOS vocabularies</a></p>
473 <table>
474 <tr>
475 <th bgcolor="#eecccc">XML Syntax</th>
476 <th width="10"/>
477 <th bgcolor="#cceecc">Turtle Syntax</th>
478 </tr>
479 <tr><td/></tr>
480 <tr>
481 <td bgcolor="#eecccc">
482 <pre>
483 &lt;skos:Concept rdf:about="#spiralGalaxy"&gt;
484 &lt;skos:prefLabel lang="en"&gt;
485 spiral galaxy
486 &lt;/prefLabel&gt;
487 &lt;skos:prefLabel lang="de"&gt;
488 Spiralgalaxie
489 &lt;/prefLabel&gt;
490 &lt;skos:altLabel lang="en"&gt;
491 spiral nebula
492 &lt;/skos:altLabel&gt;
493 &lt;skos:hiddenLabel lang="en"&gt;
494 spiral glaxy
495 &lt;/hiddenLabel&gt;
496 &lt;skos:definition lang="en"&gt;
497 A galaxy having a spiral structure.
498 &lt;/skos:definition&gt;
499 &lt;skos:scopeNote lang="en"&gt;
500 Spiral galaxies fall into one of
501 three catagories: Sa, Sc, and Sd.
502 &lt;/skos:scopeNote&gt;
503 &lt;skos:narrower
504 rdf:resource="#barredSpiralGalaxy"/&gt;
505 &lt;skos:broader
506 rdf:resource="#galaxy"/&gt;
507 &lt;skos:related
508 rdf:resource="#spiralArm"/&gt;
509 &lt;/skos:Concept&gt;
510 </pre>
511 </td>
512 <td/>
513 <td bgcolor="#cceecc">
514 <pre>
515 &lt;#spiralGalaxy&gt; a skos:Concept;
516 skos:prefLabel
517 "spiral galaxy"@en,
518 "Spiralgalaxie"@de;
519 skos:altLabel "spiral nebula"@en;
520 skos:hiddenLabel "spiral glaxy"@en;
521 skos:definition """A galaxy having a
522 spiral structure."""@en;
523 skos:scopeNote """Spiral galaxies fall
524 into one of three categories:
525 Sa, Sc, and Sd"""@en;
526 skos:narrower &lt;#barredSpiralGalaxy&gt;;
527 skos:broader &lt;#galaxy&gt;;
528 skos:related &lt;#spiralArm&gt; .
529 </pre>
530 </td>
531 </tr>
532 </table>
533 </center>
534
535 <p>A SKOS vocabulary includes the following features.</p>
536
537 <ul>
538
539 <li>A single URI representing the concept, mainly for use by computers.
540 <!--
541 <code>&lt;#spiralGalaxy&gt; a skos:Concept</code>.
542 <code>&lt;skos:Concept rdf:about="#spiralGalaxy"&gt;</code>
543 -->
544 </li>
545
546 <li>A single prefered label in each supported language of the
547 vocabulary, for use by humans.
548 <!--
549 <code>skos:prefLabel "spiral galaxy"@en, "Spiralgalaxie"@de</code>.
550 <code>&lt;skos:prefLabel&gt;spiral galaxy&lt;/skos:prefLabel&gt;</code>
551 -->
552 </li>
553
554 <li>Optional alternative labels which applications may encounter or in
555 common use, whether simple synonyms or commonly-used aliases,
556 e.g. <q>GRB</q> for "gamma-ray burst", or <q>Spiral nebula</q> for
557 spiral galaxies.
558 <!--
559 <code>skos:altLabel "GRB"@en</code>
560 <code>&lt;skos:altLabel lang="de"&gt;Spiralgalaxie&lt;/skos:altLabel&gt;</code>
561 -->
562 </li>
563
564 <li>Optional hidden labels which capture terms which are sometimes
565 used for the corresponding concept, but which are deprecated in some
566 sense. This might include common misspellings for
567 either the preferred or alternate labels, for example <q>glaxy</q> for
568 <q>galaxy</q>.
569 </li>
570
571 <li>A definition for the concept, where one exists in the original
572 vocabulary, to clarify the meaning of the term.
573 <!--
574 <code>skos:definition "A galaxy having a spiral structure."@en</code>
575 <code>&lt;skos:definition lang="en"&gt;<br/>A galaxy having a spiral structure.<br/>&lt;/skos:definition&gt;</code>
576 -->
577 </li>
578
579 <li>A scope note to further clarify a definition, or the usage of the
580 concept.
581 <!--
582 <code>skos:scopeNote "Spiral galaxies fall into one of three categories: Sa, Sc, and Sd"@en</code>
583 <code>&lt;skos:scopeNote lang="en"&gt;<br/>Spiral galaxies fall into one of three catagories: Sa, Sc, and Sd.<br/>&lt;/skos:scopeNote&gt;</code>
584 -->
585 </li>
586
587 <li>Optionally, a concept may be involved in any number of relationships
588 to other concepts. The types of relationships are
589 <ul>
590 <li>Narrower or more specific concepts, for example a link to the concept
591 representing a <q>barred spiral galaxy</q>.
592 <!--
593 <code>skos:narrower &lt;#barredSpiralGalaxy&gt;</code>.
594 <code>&lt;skos:narrower rdf:resource="#barredSpiralGalaxy"&gt;</code>
595 -->
596 </li>
597 <li>Broader or more general concepts, for example a link to the token
598 representing galaxies in general.
599 <!--
600 <code>skos:broader &lt;#galaxy&gt;</code>.
601 <code>&lt;skos:broader rdf:resource="#galaxy"&gt;</code>
602 -->
603 </li>
604 <li>Related concepts, for example a link to the token representing spiral
605 arms of galaxies
606 <!--
607 <code>skos:related &lt;#spiralArm&gt;</code>
608 <code>&lt;skos:related rdf:resource="#spiralArm"&gt;</code>
609 -->
610 <br/>
611 (note this relationship does not say that spiral galaxies have spiral
612 arms – that would be ontological information of a higher order which
613 is beyond the requirements for information stored in a vocabulary).</li>
614 </ul>
615 </li>
616 </ul>
617
618 <p>In addition to the information about a single concept, a vocabulary
619 can contain information to help users navigate its structure and
620 contents:</p>
621 <ul>
622 <li>The <q>top concepts</q> of the vocabulary, i.e. those that occur
623 at the top of the vocabulary hierarchy defined by the broader/narrower
624 relationships, can be explicitly stated to make it easier to navigate
625 the vocabulary.</li>
626
627 <li>Concepts that form a natural group can be defined as being members
628 of a <q>collection</q>.</li>
629
630 <li>Versioning information can be added using change notes.</li>
631
632 <li>Additional metadata about the vocabulary, e.g. the publisher, may
633 be documented using the Dublin Core metadata set <span class='cite'
634 >std:dublincore</span>.</li>
635 </ul>
636 </div>
637
638
639 <div class='section'>
640 <p class='title'>Relationships Between Vocabularies</p>
641
642 <p>
643 There already exist several vocabularies in the domain of astronomy.
644 Instead of attempting to replace all these existing vocabularies,
645 which have been developed to achieve different aims and user groups,
646 we embrace them.
647 This requires a mechanism to relate the concepts in the different
648 vocabularies.
649 The W3C are in the process of developing a standard for relating the
650 concepts in different SKOS vocabularies <span
651 class='cite'>std:skosMapping</span> and when completed this should be
652 reviewed for use by the IVOA.
653 </p>
654
655 <p>
656 Four types of relationship are sufficient to capture the relationships
657 between concepts in vocabularies and are similar to those defined for
658 relationships between concepts within a single vocabulary.
659 The relationships are as follows.
660 <span class='todo'>[TODO] Add specifics to the examples.</span>
661 </p>
662 <ul>
663
664 <li>
665 Equivalence between concepts, i.e. the concepts in the different
666 vocabularies refer to the same real world entity.
667 This is captured with the following RDF statement
668 <code>iau93:#SPIRALGALAXY map:exactMatch ivoat:#spiralGalaxy</code>
669 which states the the spiral galaxy concept in the IAU thesaurus is the
670 same as the spiral galaxy concept in the IVOAT.
671 (Note the use of an external namespaces <code>iau93</code> and
672 <code>ivoat</code> which must be defined within the document.)
673 </li>
674
675 <li>
676 Broader concept, i.e. there is not an equivalent concept but there is
677 a more general one.
678 This is captured with the RDF statement <code>iau93:#XXX
679 map:broadMatch ivoat:#YYY</code> which states that the IVOAT concept
680 YYY is more general than the IAU93 concept XXX.
681 </li>
682
683 <li>
684 Narrower concept, i.e. there is not an equivalent concept but there is
685 a more specific one.
686 This is captured with the RDF statement <code>iau93:#XXX
687 map:narrowMatch ivoat:#YYY</code> which states that the IVOAT concept
688 YYY is more specific than the IAU93 concept XXX.
689 </li>
690
691 <li>
692 Related concept, i.e. there is some form of relationship.
693 This is captured with the RDF statement <code>iau93:#XXX
694 map:relatedMatch ivoat:#YYY</code> which states that the IAU93 concept
695 XXX has an association with the IVOAT concept YYY.
696 </li>
697
698 </ul>
699
700 <p>
701 <span class='todo'>[TODO:] Enter text regarding the resolution of <a
702 href="http://code.google.com/p/volute/issues/detail?id=7">Issue
703 7</a>.</span>
704 </p>
705
706 </div>
707
708 </div>
709
710 <div class='section' id='publishing'>
711 <p class='title'>Publishing vocabularies (normative)</p>
712
713 <div class='section' id='pubreq'>
714 <p class='title'>Requirements</p>
715
716 <p>A vocabulary which conforms to this IVOA standard has the following
717 features. In this section, the keywords
718 <span class='rfc2119' >must</span>,
719 <span class='rfc2119' >should</span>
720 and so on, are to be interpreted as described in <span
721 class='cite'>std:rfc2119</span>.</p>
722
723 <div class='section'>
724 <p class='title'>Dereferenceable namespace</p>
725
726 <p>The namespace of the
727 vocabulary <span class='rfc2119'>must</span> be dereferenceable on the
728 web. That is, typing the namespace URL into a web browser will
729 produce human-readable documentation about the vocabulary. In
730 addition, the namespace URL <span class='rfc2119' >should</span>
731 return the RDF version of the vocabulary if it is retrieved with an
732 HTTP Accept header of <code>application/rdf+xml</code>.</p>
733
734 <p><em>Rationale: These prescriptions are intended to be compatible
735 with the patterns described in <span class='cite'>berrueta08</span>
736 and <span class='cite'>sauermann07</span>, and vocabulary distributors
737 <span class='rfc2119' >should</span> follow these patterns where
738 possible.</em></p>
739 </div>
740
741 <div class='section'>
742 <p class='title'>Long-term availability</p>
743
744 <p>The files defining a
745 vocabulary, including those of superseded versions, should remain
746 permanently available. There is no requirement that the namespace
747 URL be at any particular location, although the IVOA web pages, or the
748 online sections of the A&amp;A journal would likely be suitable
749 archival locations.</p>
750 </div>
751
752 <div class='section'>
753 <p class='title'>Distribution format</p>
754
755 <p>Vocabularies <span class='rfc2119'>must</span> be made available
756 for distribution as SKOS RDF files, in either RDF/XML <span
757 class='cite'>std:rdfxml</span> or Turtle <span
758 class='cite'>std:turtle</span> format; vocabularies <span
759 class='rfc2119'>should</span> be made available in both formats. See
760 issue <a href='@DISTURI@/issues#distformat-2'>[distformat-2]</a>.</p>
761
762 <p>A publisher <span class='rfc2119'>may</span> make available
763 documentation and supporting files in other formats.</p>
764
765 <p><em>Rationale: this does imply that the vocabulary source files can only
766 realistically be parsed using an RDF parser. An alternative is to
767 require that vocabularies be distributed using a subset of RDF/XML
768 which can also be naively handled as traditional XML; however as well
769 as creating an extra standardisation requirement, this would make it
770 effectively infeasible to write out the distribution version of the
771 vocabulary using an RDF or general SKOS tool.</em></p>
772 </div>
773
774 <div class='section'>
775 <p class='title'>Clearly versioned vocabulary</p>
776
777 <p><span class='todo' >To be decided. There are interactions with
778 'long-term availability' and 'dereferenceable namespace', since this
779 implies that the vocabulary version should be manifestly encoded in
780 the namespace URI.</span> See issue <a
781 href='@DISTURI@/issues#versioning-3' >[versioning-3]</a>.</p>
782
783 </div>
784
785 <div class='section'>
786 <p class='title'>No restrictions on source files</p>
787
788 <p>This standard does not place any restrictions on the format of the
789 files managed by the maintenance process, as long as the distributed
790 files are as specified above. See issue
791 <a href='@DISTURI@/issues#masterformat-1' >[masterformat-1]</a>.</p>
792 </div>
793
794 </div>
795
796 <div class='section' id='practices'>
797 <p class='title'>Suggested good practices</p>
798
799 <p>This standard imposes a number of requirements on conformant
800 vocabularies (see <span class='xref' >publishing</span>). In
801 this section we list a number of good practices that IVOA vocabularies
802 <span class='rfc2119'>should</span> abide by. Some of the
803 prescriptions below are more specific than good-practice guidelines
804 for vocabularies in general.</p>
805
806 <p>The adoption of the following guidelines will make it easier to use
807 vocabularies in generic VO applications. However, VO applications
808 <span class='rfc2119'>should</span> be able to accept any vocabulary
809 that complies with the latest SKOS standard
810 <span class="cite">std:skoscore</span> (this does not imply, of
811 course, that an application will necessarily understand the terms in
812 an alien vocabulary, although the presence of mappings to a known
813 vocabulary should allow it to derive some benefit).</p>
814
815 <ol>
816
817 <li>Concept identifiers <span class='rfc2119'>should</span> consist
818 only of the letters a-z, A-Z, and numbers 0-9, i.e. no spaces, no
819 exotic letters (e.g. umlauts), and no characters which would make a
820 token inexpressible as part of a URI; since tokens are for use by
821 computers only, this is not a big restriction, since the exotic
822 letters can be used within the labels and documentation if
823 appropriate.</li>
824
825 <li>The concept identifiers <span class='rfc2119'>should</span> be
826 kept in human-readable form, directly reflect the implied meaning, and
827 not be semi-random identifiers only (for example, use
828 <q>spiralGalaxy</q>, not "t1234567"); tokens <span
829 class='rfc2119'>should</span> preferably be created via a direct
830 conversion from the preferred label via removable/translation of
831 non-token characters (see above) and sub-token separation via
832 capitalisation of the first sub-token character (e.g. the label "My
833 favourite idea-label #42" is converted into
834 "MyFavouriteIdeaLabel42").</li>
835
836 <li>Labels <span class='rfc2119'>should</span> be in the form of the source vocabulary. When
837 developing a new vocabulary the singular form <span
838 class='rfc2119'>should</span> be preferred,
839 e.g. <q>spiral galaxy</q>, not "spiral galaxies". <span
840 class='todo'><a
841 href="http://code.google.com/p/volute/issues/detail?id=1">Open
842 issue</a></span></li>
843
844 <li>Each concept <span class='rfc2119'>should</span> have a definition
845 (<code>skos:definition</code>) that constitutes a short description of
846 the concept which could be adopted by an application using the
847 vocabulary. Each concept <span class='rfc2119'>should</span> have
848 additional documentation using SKOS Notes or
849 Dublin Core terms as appropriate
850 (see <span class='cite'>std:skoscore</span>)</li>
851
852 <li>The language localisation <span class='rfc2119'>should</span> be
853 declared where appropriate, in preferred labels, alternate labels,
854 definitions, and the like.</li>
855
856 <li>Relationships (<q>broader</q>, <q>narrower</q>, <q>related</q>)
857 between concepts <span class='rfc2119'>should</span> be present, but
858 are not required; if used, they <span class='rfc2119'>should</span> be
859 complete (thus all <q>broader</q> links have corresponding
860 <q>narrower</q> links in the referenced entries and <q>related</q>
861 entries link each other).</li>
862
863 <li><q>TopConcept</q> entries (see above) <span
864 class='rfc2119'>should</span> be declared and normally consist of
865 those concepts that do not have any <q>broader</q> relationships
866 (i.e. not at a sub-ordinate position in the hierarchy).</li>
867
868 <li>Publishers <span class='rfc2119'>should</span> publish
869 <q>mappings</q> between their vocabularies and other commonly used
870 vocabularies. These <span class='rfc2119'>should</span> be external to
871 the defining vocabulary document so that the vocabulary can be used
872 independently of the publisher's mappings. <span class='todo' ><a
873 href='http://code.google.com/p/volute/issues/detail?id=8' >Open
874 issue</a></span>.</li>
875 </ol>
876
877 <!--
878 <p>These suggestions are by no means trivial – there was
879 considerable discussion within the semantic working group on many of
880 these topics, particularly about token formats (some wanted lower-case
881 only), and singular versus plural forms of the labels (different
882 traditions exist within the international library science
883 community). Obviously, no publisher of an astronomical vocabulary has
884 to adopt these rules, but the adoption of these rules will make it
885 easier to use the vocabularly in external generic VO
886 applications. However, VO applications should be developed to accept
887 any vocabulary that complies with the latest SKOS standard <span
888 class="cite">std:skoscore</span>.</p>
889 -->
890 </div>
891
892 </div>
893
894
895 <div class="section" id='distvocab'>
896 <p class="title">Example vocabularies</p>
897
898 <p>The intent of having the IVOA adopt SKOS as the preferred format for
899 astronomical vocabularies is to encourage the creation and management
900 of diverse vocabularies by competent astronomical groups, so that
901 users of the VO and related resources can benefit directly and
902 dynamically without the intervention of the IAU or IVOA. However, we
903 felt it important to provide several examples of vocabularies in the
904 SKOS format as part of the proposal, to illustrate their simplicity
905 and power, and to provide an immediate vocabulary basis for VO
906 applications.</p>
907
908 <p>See also issue
909 <a href='@DISTURI@/issues#vocabset-5' >[vocabset-5]</a>. The
910 identification of sections as normative or informative depends on the
911 outcome of this issue.</p>
912
913 <p>We provide a set of SKOS files representing the vocabularies which
914 have been developed, and mappings between them. These can be
915 downloaded at the URL</p>
916 <blockquote>
917 <span class='url'>@BASEURI@/@DISTNAME@.tar.gz</span>
918 </blockquote>
919 <p class='todo'>Not yet: instead go to
920 <span class='url'>http://code.google.com/p/volute/downloads/list</span></p>
921
922 <p><span class='todo' >[To be expanded:] there are no mappings at the
923 moment. Also, the vocabularies are all in a single language, though
924 translations of the IAU93 thesaurus are available. See also
925 <a href='http://code.google.com/p/volute/issues/detail?id=8' >issue 8</a></span></p>
926
927 <div class='section' id='vocab-constellation'>
928 <p class='title'>A Constellation Name Vocabulary</p>
929
930 <p>This vocabulary is presented as a simple example of an astronomical vocabulary for a very particular purpose, e.g. handling constellation information like that commonly encountered in variable star research. For example, <q>SS Cygni</q> is a cataclysmic variable located in the constellation <q>Cygnus</q>. The name of the star uses the genitive form <q>Cygni</q>, but the alternate label <q>SS Cyg</q> uses the standard abbreviation <q>Cyg</q>. Given the constellation vocabulary, all of these forms are recorded together in a computer-manipulatable format. Various incorrect forms should probably be represented in SKOS `hidden labels'</p>
931
932 <p>The &lt;skos:ConceptScheme&gt; contains a single &lt;skos:TopConcept&gt;, <q>constellation</q></p>
933 <br/><br/><center>
934 <table>
935 <tr><th bgcolor="#eecccc">XML Syntax</th>
936 <th width="10"/><th bgcolor="#cceecc">Turtle Syntax</th></tr>
937 <tr><td/></tr>
938 <tr>
939 <td bgcolor="#eecccc">
940 <pre>
941 &lt;skos:Concept rdf:about="#constellation"&gt;
942 &lt;skos:inScheme rdf:resource=""/&gt;
943 &lt;skos:prefLabel&gt;
944 constellation
945 &lt;/skos:prefLabel&gt;
946 &lt;skos:definition&gt;
947 IAU-sanctioned constellation names
948 &lt;/skos:definition&gt;
949 &lt;skos:narrower rdf:resource="#Andromeda"/&gt;
950 ...
951 &lt;skos:narrower rdf:resource="#Vulpecula"/&gt;
952 &lt;/skos:Concept&gt;
953 </pre>
954 </td>
955 <td/>
956 <td bgcolor="#cceecc">
957 <pre>
958 &lt;#constellation&gt; a :Concept;
959 :inScheme &lt;&gt;;
960 :prefLabel "constellation";
961 :definition "IAU-sanctioned constellation names";
962 :narrower &lt;#Andromeda&gt;;
963 ...
964 :narrower &lt;#Vulpecula&gt;.
965 </pre>
966 </td></tr>
967 </table></center>
968 <p>and the entry for <q>Cygnus</q> is</p>
969 <center><table><tr>
970 <td bgcolor="#eecccc">
971 <pre>
972 &lt;skos:Concept rdf:about="#Cygnus"&gt;
973 &lt;skos:inScheme rdf:resource=""/&gt;
974 &lt;skos:prefLabel&gt;Cygnus&lt;/skos:prefLabel&gt;
975 &lt;skos:definition&gt;Cygnus&lt;/skos:definition&gt;
976 &lt;skos:altLabel&gt;Cygni&lt;/skos:altLabel&gt;
977 &lt;skos:altLabel&gt;Cyg&lt;/skos:altLabel&gt;
978 &lt;skos:broader rdf:resource="#constellation"/&gt;
979 &lt;skos:scopeNote&gt;
980 Cygnus is nominative form; the alternative
981 labels are the genitive and short forms
982 &lt;/skos:scopeNote&gt;
983 &lt;/skos:Concept&gt;
984 </pre>
985 </td>
986 <td width="10"/>
987 <td bgcolor="#cceecc">
988 <pre>
989 &lt;#Cygnus&gt; a :Concept;
990 :inScheme &lt;&gt;;
991 :prefLabel "Cygnus";
992 :definition "Cygnus";
993 :altLabel "Cygni";
994 :altLabel "Cyg";
995 :broader &lt;#constellation&gt;;
996 :scopeNote """Cygnus is nominative form;
997 the alternative labels are the genitive and
998 short forms""" .
999 </pre>
1000 </td>
1001 </tr></table></center>
1002
1003 <p>Note that SKOS alone does not permit the distinct differentiation
1004 of genitive forms and abbreviations, but the use of alternate labels
1005 is more than adequate enough for processing by VO applications where
1006 the difference between <q>SS Cygni</q>, <q>SS Cyg</q>, and the incorrect form
1007 <q>SS Cygnus</q> is probably irrelevant.</p>
1008 </div>
1009
1010 <div class='section' id='vocab-aa'>
1011 <p class='title'>The Astronomy &amp; Astrophysics Keyword List</p>
1012
1013 <p>
1014 This vocabulary is a set of keywords made available on a web page by
1015 the publisher of the journal.
1016 The intended usage of the vocabulary is to tag articles with
1017 descriptive keywords to aid searching for articles on a particular
1018 topic.
1019 </p>
1020
1021 <p>
1022 The keywords are organised into categories which have been modelled as
1023 hierarchical relationships.
1024 Additionally, some of the keywords are grouped into collections which
1025 has been mirrored in the SKOS version.
1026 The vocabulary contains no definitions, alternative labels, scope
1027 notes, or related links, as these are not provided in the original
1028 keyword list.
1029 </p>
1030
1031 </div>
1032
1033 <div class='section' id='vocab-aoim'>
1034 <p class='title'>The AOIM Taxonomy</p>
1035
1036 <p>
1037 This vocabulary is published by the IVOA to allow images to be tagged
1038 with keywords that are relevant for the public.
1039 It consists of a set of keywords organised into an enumerated
1040 hierarchical structure.
1041 Each term consists of a taxonomic number and a label.
1042 There are no alternative labels, definitions, scope notes, or cross
1043 references.
1044 </p>
1045
1046 <p>When converting the AOIM into SKOS, it was decided to model the
1047 taxonomic number as an alternative label.
1048 Since there are duplication of terms, the token for a term consists of
1049 the full hierarchical location of the term.
1050 Thus, it is possible to distinguish between</p>
1051 <pre>
1052 Planet -> Feature -> Surface -> Canyon
1053 </pre>
1054 <p>and</p>
1055 <pre>
1056 Planet -> Satellite -> Feature -> Surface -> Canyon
1057 </pre>
1058 <p>which have the tokens <code>PlanetFeatureSurfaceCanyon</code> and
1059 <code>PlanetSatelliteFeatureSurfaceCanyon</code> respectively.
1060 </p>
1061
1062 </div>
1063
1064 <div class='section' id='vocab-ucd1'>
1065 <p class='title'>The UCD1+ Vocabulary</p>
1066
1067 <p>The UCD standard is an officially sanctioned and managed vocabulary
1068 of the IVOA. The normative document is a simple text file containing
1069 entries consisting of tokens (e.g. <code>em.IR</code>), a short
1070 description, and usage information (<q>syntax codes</q> which permit
1071 UCD tokens to be concatenated). The form of the tokens implies a
1072 natural hierarchy: <code>em.IR.8-15um</code> is obviously a narrower
1073 term than <code>em.IR</code>, which in turn is narrower than
1074 <code>em</code>.</p>
1075
1076 <p>Given the structure of the UCD1+ vocabulary, the natural
1077 translation to SKOS consists of preferred labels equal to the original
1078 tokens (the UCD1 words include dashes and periods), vocabulary tokens
1079 created using guidelines in <span class='xref'
1080 >practices</span> (e.g., "emIR815Um" for
1081 <code>em.IR.8-15um</code>), direct use of the definitions, and the syntax codes
1082 placed in usage documentation: <code>&lt;skos:scopeNote&gt;UCD syntax code: P&lt;/skos:scopeNote&gt;</code></p>
1083
1084 <p>Note that the SKOS document containing the UCD1+ vocabulary does
1085 NOT consistute the official version: the normative document is still
1086 the text list. However, on the long term, the IVOA may decide to make
1087 the SKOS version normative, since the SKOS version contains all of the
1088 information contained in the original text document but has the
1089 advantage of being in a standard format easily read and used by any
1090 application on the semantic web.</p>
1091
1092 </div>
1093
1094 <div class='section' id='vocab-iau93'>
1095 <p class='title'>The 1993 IAU Thesaurus</p>
1096
1097 <p>The IAU Thesaurus consists of concepts with mostly capitalised
1098 labels and a rich set of thesaurus relationships (<q>BF</q> for
1099 "broader form", <q>NF</q> for <q>narrower form</q>, and <q>RF</q> for
1100 <q>related form</q>). The thesaurus also contains <q>U</q> (for
1101 <q>use</q>) and <q>UF</q> (<q>use for</q>) relationships. In a SKOS
1102 model of a vocabulary these are captured as alternative labels. A
1103 separate document contains translations of the vocabulary terms in
1104 five languages: English, French, German, Italian, and
1105 Spanish. Enumerable concepts are plural (e.g. <q>SPIRAL
1106 GALAXIES</q>) and non-enumerable concepts are singular
1107 (e.g. <q>STABILITY</q>). Finally, there are some usage hints like
1108 <q>combine with other</q></p>
1109
1110 <p>In converting the IAU Thesaurus to SKOS, we have been as faithful
1111 as possible to the original format of the thesaurus. Thus, preferred
1112 labels have been kept in their uppercase format.</p>
1113
1114 <p>The IAU Thesaurus has been unmaintained since its initial production in
1115 1993; it is therefore significantly out of date in places. This
1116 vocabulary is published for the sake of completeness, and to make the
1117 link between the evolving vocabulary work and any uses of the 1993
1118 vocabulary which come to light. We do not expect to make any future
1119 maintenance changes to this vocabulary, and would expect the IVOAT
1120 vocabulary, based on this one, to be used instead (see <span class='xref'>vocab-ivoat</span>).</p>
1121
1122 </div>
1123
1124 <div class='section' id='vocab-ivoat'>
1125 <p class='title'>Towards an IVOA Thesaurus</p>
1126
1127 <p>While it is true that the adoption of SKOS will make it easy to
1128 publish and access different astronomical vocabularies, the fact is
1129 that there is no vocabulary which makes it easy to jump-start the
1130 use of vocabularies in generic astrophysical VO applications: each of
1131 the previously developed vocabularies has their own limits and
1132 biases. For example, the IAU Thesaurus provides a large number of
1133 entries, copious relationships, and translations to four other languages,
1134 but there are no definitions, many concepts are now only useful for
1135 historical purposes (e.g. many photographic or historical instrument
1136 entries), some of the relationships are false or outdated, and many
1137 important or newer concepts and their common abbreviations are
1138 missing.</p>
1139
1140 <p>Despite its faults, the IAU Thesaurus constitutes a very extensive
1141 vocabulary which could easily serve as the basis vocabulary once
1142 we have removed its most egregious faults and extended it to cover the
1143 most obvious semantic holes. To this end, a heavily revised IAU
1144 thesaurus is in preparation for use within the IVOA and other
1145 astronomical contexts. The goal is to provide a general vocabulary
1146 foundation to which other, more specialised, vocabularies can be added
1147 as needed, and to provide a good <q>lingua franca</q> for the creation of
1148 vocabulary mappings.</p>
1149 </div>
1150 </div> <!-- End: Example vocabularies -->
1151
1152
1153 <div class="appendices">
1154
1155 <div class="section-nonum" id="bibliography">
1156 <p class="title">Bibliography</p>
1157 <?bibliography rm-refs ?>
1158 </div>
1159
1160 <p style="text-align: right; font-size: x-small; color: #888;">
1161 $Revision$ $Date$
1162 </p>
1163
1164 </div>
1165
1166 </body>
1167 </html>

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msdemlei@ari.uni-heidelberg.de
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