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Re: Clarification required: BP6 "use HTTP URIs for spatial things"

From: Joshua Lieberman <jlieberman@tumblingwalls.com>
Date: Wed, 31 Aug 2016 11:45:56 -0400
Cc: Frans Knibbe <frans.knibbe@geodan.nl>, SDW WG Public List <public-sdw-wg@w3.org>
Message-Id: <EBC89211-858E-4B22-AC53-ECE3AB7DF427@tumblingwalls.com>
To: Jeremy Tandy <jeremy.tandy@gmail.com>

> On Aug 31, 2016, at 10:51 AM, Jeremy Tandy <jeremy.tandy@gmail.com> wrote:
> 
> @josh- following brief discussion on the call and re-reading this just now I understand your point better.
> 
> The "spatial things" are representations; for example, "the lighthouse in the real world seen as a vertical obstruction" and "the lighthouse in the real world seen as a maritime navigation aid" - and it takes some additional assessment to determine that the "lighthouse in the real world" for both is, in fact, the same lighthouse - this is what we've described as "reconciliation". 
> 
> My problem is that in RDF Schema [1], as far as I understand, classes are all about sets and set membership. So Eddystone lighthouse can be a member both classes because it has the requisite properties that qualify it as a "vertical obstruction" and a "maritime navigation aid". What I have seen in Linked Data is that folks don't bother to identify the _representation_, they identify the thing that is being represented. So in the case that different authorities mint different URIs for Eddystone Lighthouse (as indicated in my example), they _both_ refer to Eddystone Lighthouse (not the representation) which means that <owl:sameAs> is appropriate (e.g. both identifiers refer to the same resource).

That’s what I’ve indicated is a possibility, but someone has to define a set which are lighthouses that are members of both other sets (classes), and then it takes on both sets of accompanying attributes and axioms, which may or may not be consistent with each other. owl:sameAs is ambiguous, so it isn’t clear that it can clearly mean “different information resources and conceptualization, but same real world entity”. The use of “sameRealWorldEntityAs” would give us a weaker, but clearer assertion.

This is only really an issue in special cases. In other cases the discerned feature is the same, but two different classes may have been defined and so both the indirect identity of the real world entity and use of owl:sameAs is clear. It’s precisely the tendency of “folks” to conflate everything together, data, geometry, concept, real world thing, that works most of the time but gets into huge trouble once in a while.

Josh

> 
> [ that makes sense to me, but I wonder if I'm the lone member of the RDF class that understands what I'm trying to say :-) ]
> 
> If this doesn't make sense - or I'm just plain wrong, then let's have a call tomorrow ... skype, hangouts or what ever takes your fancy.
> 
> @frans-
> 
> > So I would need to have a different URI for my triangle [and my SVG file].
> 
> Yes. I guess that the triangle might be <http://example.com/shapes/my-triangle <http://example.com/shapes/my-triangle>> and the SVG file <http://example.com/shapes/my-triangle.svg <http://example.com/shapes/my-triangle.svg>>; some HTTP server config might be used to serve the SVG file when a user agent requests <http://example.com/shapes/my-triangle <http://example.com/shapes/my-triangle>>.  
> 
> Regarding Req. 5.45 "subject equality" [2] ("There should be a recommended way to express that data based on different models are about the same real world spatial thing."), I think that both approaches would suffice. @josh's approach retains a little more indirection, mine seems to follow what I see in Linked Data practice.
> 
> [1]: https://www.w3.org/TR/rdf-schema/ <https://www.w3.org/TR/rdf-schema/> 
> [2]: http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SubjectEquality <http://w3c.github.io/sdw/UseCases/SDWUseCasesAndRequirements.html#SubjectEquality> 
> 
> On Wed, 31 Aug 2016 at 14:40 Joshua Lieberman <jlieberman@tumblingwalls.com <mailto:jlieberman@tumblingwalls.com>> wrote:
> Jeremy,
> 
> So as representations, these are not “owl:sameAs”. We assume that as feature data, each refers to a real world entity, but we don’t assert that this VerticalObstruction is the same individual as this MaritimeNavigationAid. We just are suspecting or asserting that the same real world thing is being discerned in two different ways. Someone may define a lighthouse class as subclassing both, otherwise a slightly specialized relation (e.g. sdwgeo:sameRealWorldEntityAs) would be useful here.
> 
> Josh
> 
>> On Aug 31, 2016, at 8:41 AM, Jeremy Tandy <jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>> wrote:
>> 
>> > That still leaves a gap in expressing whether two feature data entities represent the same real world entity. Perhaps we need a "sameFeatureAs" predicate to address this.
>> 
>> @josh - can we clarify my understanding please?
>> 
>> In the BP doc §4 "Spatial things, features and geometry" [1] I use a lighthouse example, so I'll continue with that ...
>> 
>> We have one real lighthouse (Eddystone Lighthouse) that is discerned as a different Type by different communities: "VerticalObstruction" and "MaritimeNavigationAid". In ISO 19100 parlance, these are two distinct feature types. The two "Features" might be encoded in GML as follows (forgive any errors in my illustrative example):
>> 
>> <VerticalObstruction gml:id="a">
>>     <gml:name>Eddystone</gml:name>
>>     <gml:identifier codeSpace="http://example.com/sar/features/vo/">EDY</gml:identifier <http://example.com/sar/features/vo/%22%3EEDY%3C/gml:identifier>>
>>     <geometry>
>>         <gml:Point gml:id="a-p1" srsDimension="2" srsName="EPSG:4326">
>>             <gml:pos>50.184 -4.268</gml:pos>
>>         </gml:Point>
>>     </geometry>
>>     <height uom="m">41</height>
>> </VerticalObstruction>
>> 
>> <MaritimeNavigationAid gml:id="b">
>>     <gml:name>Eddystone Lighthouse</gml:name>
>>     <gml:identifier codeSpace="http://example.org/maritime/navaid/ <http://example.org/maritime/navaid/>">2650253</gml:identifier>
>>     <geo>
>>         <gml:Point gml:id="b-p1" srsDimension="2" srsName="EPSG:4326">
>>             <gml:pos>50.2 -4.3</gml:pos>
>>         </gml:Point>
>>     </geo>
>>     <lightCharacteristic>
>>         ...
>>     </lightCharacteristic>
>> </MaritimeNavigationAid>
>> 
>> So we have two Features (which we collectively have agreed are "spatial things"), with identifiers <http://example.com/sar/features/vo/EDY <http://example.com/sar/features/vo/EDY>> and <http://example.org/maritime/navaid/2650253 <http://example.org/maritime/navaid/2650253>>. Respectively, the XML elements that describe these features are identified as "a" and "b" using the @gml:id attribute.
>> 
>> If we are using "indirect identification" then _both_ <http://example.com/sar/features/vo/EDY <http://example.com/sar/features/vo/EDY>> and <http://example.org/maritime/navaid/2650253 <http://example.org/maritime/navaid/2650253>> are treated as identifiers for the _real_ Eddystone Lighthouse; we simply don't care to differentiate between the real world thing and the information record. In which case, <owl:sameAs>  would seem sufficient? The "height" and "lightCharacteristic" properties are both applicable to the real Eddystone Lighthouse. Some judgement would be required to decide which point geometry ("geo" or "geometry" property) is considered "best".
>> 
>> The way I think about it, @gml:id is more like the identifier for a named graph; a container for a set of properties ... 
>> 
>> Am I missing something???
>> 
>> Jeremy
>> 
>> 
>> [1]: http://w3c.github.io/sdw/bp/#spatial-things-features-and-geometry <http://w3c.github.io/sdw/bp/#spatial-things-features-and-geometry> 
>> 
>> On Wed, 31 Aug 2016 at 12:42 Joshua Lieberman <jlieberman@tumblingwalls.com <mailto:jlieberman@tumblingwalls.com>> wrote:
>> If we are asserting that spatial data on the Web is "always" feature data that represents a real world entity, then yes, we don't have the general Web "is it or isn't it physical" ambiguity and can assume that a feature data identifier also and indirectly identifies the feature. That still leaves a gap in expressing whether two feature data entities represent the same real world entity. Perhaps we need a "sameFeatureAs" predicate to address this.
>> 
>> Josh
>> 
>> Joshua Lieberman, Ph.D.
>> Principal, Tumbling Walls Consultancy
>> Tel/Direct: +1 617-431-6431
>> jlieberman@tumblingwalls.com <mailto:jlieberman@tumblingwalls.com>
>> 
>> On Aug 31, 2016, at 07:29, Frans Knibbe <frans.knibbe@geodan.nl <mailto:frans.knibbe@geodan.nl>> wrote:
>> 
>>> Hello,
>>> 
>>> As stated before, I don't think the httpRange-14 problem exists in our domain of discourse. I think (and hope) that confusion can only occur when the things that are described are digital things, or things that can be transmitted over a computer network, like web pages or mail boxes. It seems to me that spatial things are never that type of thing. Therefore there is no reason to take precautions against possible confusion. 
>>> 
>>> That probably means +1.
>>> 
>>> Greetings,
>>> Frans
>>> 
>>>  
>>> 
>>> On 31 August 2016 at 09:50, Jeremy Tandy <jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>> wrote:
>>> Thanks Rob & Clemens ...
>>> 
>>> On Wed, 31 Aug 2016 at 08:30, Clemens Portele <portele@interactive-instruments.de <mailto:portele@interactive-instruments.de>> wrote:
>>> +1
>>> 
>>> 
>>> On 30 August 2016 at 10:10:26, Jeremy Tandy (jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>) wrote:
>>> 
>>>> Hi. It would be good to close this issue out & include our collective recommendation in the BP doc working draft.
>>>> 
>>>> PROPOSAL: SDW working group recommends use of "indirect identifiers" for spatial things
>>>> 
>>>> ... I'll start the voting.
>>>> 
>>>> +1
>>>> 
>>>> Jeremy
>>>> 
>>>> (BTW, to make sense of the PROPOSAL you'll need to read the email thread)
>>>> 
>>>> On Fri, 26 Aug 2016 at 10:12 Linda van den Brink <l.vandenbrink@geonovum.nl <mailto:l.vandenbrink@geonovum.nl>> wrote:
>>>> So… do we agree we can recommend indirect identifiers, or do we try to fix the issue with getting the correct identifier as Rob describes?
>>>> 
>>>>  
>>>> While waiting for this I’ve updated the issue and the text referring to the issue in BP6.
>>>> 
>>>>  
>>>> Van: Rob Atkinson [mailto:rob@metalinkage.com.au <mailto:rob@metalinkage.com.au>]
>>>> Verzonden: woensdag 24 augustus 2016 13:56
>>>> Aan: Jeremy Tandy; Phil Archer; Linda van den Brink; Bill Roberts
>>>> 
>>>> 
>>>> CC: SDW WG Public List
>>>> 
>>>> Onderwerp: Re: Clarification required: BP6 "use HTTP URIs for spatial things"
>>>> 
>>>>  
>>>> Hi
>>>> 
>>>>  
>>>> Agree this is a real concern - people cant be blamed for doing the obvious, if dumb, thing..
>>>> 
>>>>  
>>>> I think we should take note of best practice in the HTML world - which is often to include a citable link to a resource in the rendered view.  Or a "share" or something similar. We can also put fairly explicit annotation in machine-readable code - stating that the resource is about the URI - and even notes saying when citing this resource use the URI....
>>>> 
>>>>  
>>>> I'd also like to see browsers evolve to offer you the original link or the redirected when cutting and pasting - how hard can it be!   
>>>> 
>>>>  
>>>> Maybe we can get Ed to ask around Google Chrome team for suggestions on how best to handle this :-)
>>>> 
>>>>  
>>>> Rob
>>>> 
>>>>  
>>>>  
>>>>  
>>>> On Wed, 24 Aug 2016 at 18:27 Jeremy Tandy <jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>> wrote:
>>>> 
>>>> Yes, I think so ... And we should do so if we are recommending "indirect identification".
>>>> 
>>>> Jeremy
>>>> 
>>>> On Wed, 24 Aug 2016 at 09:24, Phil Archer <phila@w3.org <mailto:phila@w3.org>> wrote:
>>>> 
>>>> Bill's comments also made me think about some of the classic arguments,
>>>> such as that a lake doesn't have a last updated date and isn't 435KB
>>>> big. Which are true, however, that kind of metadata generally comes from
>>>> the server, i.e. the HTTP layer. That's an over simplification but the
>>>> point is that it is relatively easy to avoid deliberately creating
>>>> misleading metadata - metadata about the doc rather than the thing it
>>>> describes - and it's also generally easy to avoid looking for that metadata.
>>>> 
>>>> Is there scope for some BP advice there?
>>>> 
>>>> Phil.
>>>> 
>>>> On 24/08/2016 08:25, Jeremy Tandy wrote:
>>>> > Thanks Linda. More clear examples where being "correct" (in terms of
>>>> > avoiding uri collisions by using two distinct uris) is making things worse
>>>> > because users take the wrong one!
>>>> >
>>>> > So, as a WG, are we content to recommend this "indirect identification"
>>>> > pattern where thing & info resource identifiers are conflated?
>>>> >
>>>> > Bill has added some good points about how to avoid impacts of uri
>>>> > collision- by using the (dataset) metadata to talk about licenses and
>>>> > creators for the information ...
>>>> > On Wed, 24 Aug 2016 at 07:52, Linda van den Brink <l.vandenbrink@geonovum.nl <mailto:l.vandenbrink@geonovum.nl>>
>>>> > wrote:
>>>> >
>>>> >> Experience from the Netherlands: we have the id/doc pattern in our URI
>>>> >> strategy, based on the Cool URIs note [8] and the ISA study on persistent
>>>> >> identifiers [9].
>>>> >>
>>>> >>
>>>> >>
>>>> >> That being said, same as Bill I also notice data users getting confused
>>>> >> and generally using the /doc/  URI as that is the one they can copy from
>>>> >> their browser address bar. This is not only casual confusion but also ends
>>>> >> up in published information resources.
>>>> >>
>>>> >>
>>>> >>
>>>> >> You see this, for example, all over the CB-NL which is a vocabulary for
>>>> >> the building sector and contains links to other Dutch standards such as
>>>> >> IMGeo, an information model and vocabulary for large scale topography. E.g.
>>>> >> the CB-NL concept of ‘Gebouw’ (Building) [10]  links to two IMGeo concepts
>>>> >> ‘Pand’ (building part) and ‘Overig Bouwwerk’ (other construction) using
>>>> >> their /doc/ URIs. If you click on Pand (which doesn’t have its own landing
>>>> >> page in CB-NL so I can’t include the link) you will see it includes the
>>>> >> /doc/  URI as the identifier of Pand.
>>>> >>
>>>> >>
>>>> >>
>>>> >> This is an example where it occurs in vocabularies, but I also see it
>>>> >> happen with identifiers for data instances.
>>>> >>
>>>> >>
>>>> >>
>>>> >> [8]: https://www.w3.org/TR/cooluris/ <https://www.w3.org/TR/cooluris/>
>>>> >>
>>>> >> [9]:
>>>> >> https://joinup.ec.europa.eu/sites/default/files/D7.1.3%20-%20Study%20on%20persistent%20URIs_0.pdf <https://joinup.ec.europa.eu/sites/default/files/D7.1.3%20-%20Study%20on%20persistent%20URIs_0.pdf>
>>>> >> 10: http://ont.cbnl.org/cb/def/Gebouw <http://ont.cbnl.org/cb/def/Gebouw>
>>>> >>
>>>> >>
>>>> >>
>>>> >> Linda
>>>> >>
>>>> >>
>>>> >>
>>>> >> *Van:* Jeremy Tandy [mailto:jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>]
>>>> >> *Verzonden:* dinsdag 23 augustus 2016 20:57
>>>> >> *Aan:* Bill Roberts
>>>> >> *CC:* SDW WG Public List
>>>> >> *Onderwerp:* Re: Clarification required: BP6 "use HTTP URIs for spatial
>>>> >> things"
>>>> >>
>>>> >>
>>>> >>
>>>> >> Thanks Bill. Sounds very coherent ... I hoped for some responses such as
>>>> >> this based on practical experience. Jeremy
>>>> >>
>>>> >> On Tue, 23 Aug 2016 at 19:41, Bill Roberts <bill@swirrl.com <mailto:bill@swirrl.com>> wrote:
>>>> >>
>>>> >> ah Jeremy, you are a brave man to poke the sleeping beast of httpRange-14.
>>>> >>
>>>> >>
>>>> >>
>>>> >> But I'll get my thoughts in early, then I can tune out of the ensuing mail
>>>> >> avalanche :-)
>>>> >>
>>>> >>
>>>> >>
>>>> >> When publishing Linked Data about places we (at Swirrl) generally do the
>>>> >> id/doc fandango, but to be honest I think data users either don't notice,
>>>> >> or they get confused by it.  In the applications we are working with (and I
>>>> >> acknowledge that others may have different applications and different
>>>> >> experiences), it wouldn't cause any problems to have a single URI, the 'id'
>>>> >> URI if you like.  We just don't find a need to say anything about the /doc/
>>>> >> URI.  If we were starting again, I'd probably ditch the /doc/ and the 303
>>>> >> and rely on context and a little bit of documentation to make it clear what
>>>> >> we mean.
>>>> >>
>>>> >>
>>>> >>
>>>> >> The place where we find a need to talk about creators and licences and
>>>> >> modified dates is in metadata about datasets where a dataset might be a
>>>> >> collection of information about a bunch of places - and we treat datasets
>>>> >> as an 'information resource'. If someone requests a dataset URI we return a
>>>> >> status code of 200 and the dataset metadata as the response.  That metadata
>>>> >> includes info on where to get all the contents of the dataset if you want
>>>> >> that.
>>>> >>
>>>> >>
>>>> >>
>>>> >> By the way, though it's sensible and consistent, I find that the implied
>>>> >> and parallel property stuff makes it more rather than less complicated.
>>>> >>
>>>> >>
>>>> >>
>>>> >> Bill
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >> On 23 August 2016 at 17:37, Jeremy Tandy <jeremy.tandy@gmail.com <mailto:jeremy.tandy@gmail.com>> wrote:
>>>> >>
>>>> >> All-
>>>> >>
>>>> >>
>>>> >>
>>>> >> Linda has done a great job of consolidating the best practices are use of
>>>> >> identifiers. We have just one [1] now.
>>>> >>
>>>> >>
>>>> >>
>>>> >> Reading though just now, it occurred to me that there's still an open
>>>> >> issue about identifier assignment ...
>>>> >>
>>>> >>
>>>> >>
>>>> >> W3C's Architecture of the World Wide Web constraint "URIs identify a
>>>> >> single resource" [2] asserts "Assign distinct URIs to distinct resources"
>>>> >> in order to avoid URI collisions [2a] which "often imposes a cost in
>>>> >> communication due to the effort required to resolve ambiguities".
>>>> >> Discussions from earlier years in UK Gov Linked Data working group (and
>>>> >> elsewhere) concluded that the "real world thing" and "information resource
>>>> >> that describes the real world thing" are separate resources. I think this
>>>> >> is based on a (purist?) view when working with RDF of needing to be totally
>>>> >> clear on "what's the subject" of each triple ... the thing or the document.
>>>> >> This manifests as URIs with `id` or `doc` included somewhere to distinguish
>>>> >> between the resources and some RDF triples to clarify that the doc resource
>>>> >> is talking about the thing resource etc..
>>>> >>
>>>> >>
>>>> >>
>>>> >> (dangerously close to "httpRange-14" [3] here ... let's avoid that bear
>>>> >> trap)
>>>> >>
>>>> >>
>>>> >>
>>>> >> Jeni Tennison's "URLs in Data Primer" draft TAG note captures this
>>>> >> practice in §5.3 "Publishing data" [4]:
>>>> >>
>>>> >>
>>>> >>
>>>> >> ```
>>>> >>
>>>> >> Publishers can help enable more accurate merging of data from different
>>>> >> sites if they support URLs for each entity
>>>> >> <https://www.w3.org/TR/urls-in-data/#dfn-entity <https://www.w3.org/TR/urls-in-data/#dfn-entity>> they or other sites may
>>>> >> wish to describe, separate from the landing pages
>>>> >> <https://www.w3.org/TR/urls-in-data/#dfn-landing-page <https://www.w3.org/TR/urls-in-data/#dfn-landing-page>> or records
>>>> >> <https://www.w3.org/TR/urls-in-data/#dfn-record <https://www.w3.org/TR/urls-in-data/#dfn-record>> that they publish.
>>>> >>
>>>> >> ```
>>>> >>
>>>> >>
>>>> >>
>>>> >> Yet Architecture of the World Wide Web §2.2.3 "Indirect identification"
>>>> >> [5] notes that:
>>>> >>
>>>> >>
>>>> >>
>>>> >> ```
>>>> >>
>>>> >> To say that the URI "mailto:nadia@example.com <mailto:nadia@example.com>" identifies both an
>>>> >> Internet mailbox and Nadia, the person, introduces a URI collision.
>>>> >> However, we can use the URI to indirectly identify Nadia. Identifiers are
>>>> >> commonly used in this way.
>>>> >>
>>>> >> ```
>>>> >>
>>>> >>
>>>> >>
>>>> >> This is consistent with what I recall TimBL saying at TPAC-2015 in regards
>>>> >> to Vcard; come the finish, no one really cares to distinguish between the
>>>> >> thing and its associated information resource.
>>>> >>
>>>> >>
>>>> >>
>>>> >> ... And in most cases, one can use context to determine whether a
>>>> >> statement concerns the thing or the information resource. In those cases
>>>> >> where you can't, "URLs in Data Primer" suggests some mechanisms to mitigate
>>>> >> such confusion [6][7].
>>>> >>
>>>> >>
>>>> >>
>>>> >> I think that in our SDW WG discussion we have concluded that we _are_
>>>> >> content to use "indirect identification" - e.g. that we use URIs that
>>>> >> conflate the thing and document resource.
>>>> >>
>>>> >>
>>>> >>
>>>> >> Please can we confirm this? Assuming that indirect identification is
>>>> >> "approved" as best practice, then it seems prudent to add a note to the BP
>>>> >> document saying "don't worry about distinguishing between thing and
>>>> >> resource; indirect identification is fine" (etc.)
>>>> >>
>>>> >>
>>>> >>
>>>> >> Thanks, Jeremy
>>>> >>
>>>> >>
>>>> >>
>>>> >> [1]: http://w3c.github.io/sdw/bp/#globally-unique-ids <http://w3c.github.io/sdw/bp/#globally-unique-ids>
>>>> >>
>>>> >> [2]: https://www.w3.org/TR/webarch/#pr-uri-collision <https://www.w3.org/TR/webarch/#pr-uri-collision>
>>>> >>
>>>> >> [2a]: https://www.w3.org/TR/webarch/#URI-collision <https://www.w3.org/TR/webarch/#URI-collision>
>>>> >>
>>>> >> [3]: https://www.w3.org/2001/tag/group/track/issues/14 <https://www.w3.org/2001/tag/group/track/issues/14>
>>>> >>
>>>> >> [4]: https://www.w3.org/TR/urls-in-data/#publishing-data <https://www.w3.org/TR/urls-in-data/#publishing-data>
>>>> >>
>>>> >> [5]: https://www.w3.org/TR/webarch/#indirect-identification <https://www.w3.org/TR/webarch/#indirect-identification>
>>>> >>
>>>> >> [6]: https://www.w3.org/TR/urls-in-data/#documenting-properties <https://www.w3.org/TR/urls-in-data/#documenting-properties>
>>>> >>
>>>> >> [7]: https://www.w3.org/TR/urls-in-data/#authoring-specifications <https://www.w3.org/TR/urls-in-data/#authoring-specifications>
>>>> >>
>>>> >>
>>>> >>
>>>> >>
>>>> >
>>>> 
>>>> --
>>>> 
>>>> 
>>>> Phil Archer
>>>> W3C Data Activity Lead
>>>> http://www.w3.org/2013/data/ <http://www.w3.org/2013/data/>
>>>> 
>>>> http://philarcher.org <http://philarcher.org/>
>>>> +44 (0)7887 767755 <tel:%2B44%20%280%297887%20767755>
>>>> @philarcher1
>>>> 
>>> 
> 


Received on Wednesday, 31 August 2016 15:46:32 UTC

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