Our store doesn't capture in RDF elements containing extension
attributes so yesterday we started climbing this hill in our trek to
Atom/OWL mountain. Ben Szekely, Lee and I were discussing how do we want
to model extensions in a way that was not as structured as Powell's
approach and came up with some good ideas.

Here's the problem child #1:

<a:link rel="replies" href="foo" thr:count="10" xmlns:thr="urn:foo" />

Here's another problem child #2:

<a:id xmlns:x="urn:bar"
x:identifierType="lsid">urn:lsid:ex.org:entries:1</a>

#1 is not that bad:

 :link [ a :Link;
           :rel iana:alternate ;
           :to [ :src <http://example.org/>;] ;

           thr:count "10"; # notice JS extension
           ];

#2 is where the hill got a little steep:

[] :id "urn:lsid:ex.org:entries:1"^^xsd:anyURI .

However, Lee remembered to check the hiking module and found this gem:

http://www.w3.org/TR/rdf-schema/#ch_value

"""rdf:value is an instance of rdf:Property that may be used in
describing structured values."""

There's a note to see example 16 in the RDF primer, but that's not
right. Instead you must go to Section 4.4

http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#rdfvalue

"""in some cases one of the parts of the structured value is often
thought of as the "main" value, with the other parts of the relation
providing additional contextual or other information that qualifies the
main value."""

The most relevant line: "...or other information that qualifies the main
value..."

[] :id [
    rdf:value "urn:lsid:ex.org:entries:1"^^xsd:anyURI

    # other contextual information..
] .

Now we can go into the extension attributes...

[] :id [
    rdf:value "urn:lsid:ex.org:entries:1"^^xsd:anyURI
    x:identifierType "lsid" ;
    random:property  "randomValue" .
] .

x:identifierType rdfs:type atom:extensionAttributeProperty;
atom:extensionAttributeProperty rdfs:subClassOf rdf:Property .

Now because of atom:extensionAttributeProperty we can figure what
extension attributes to attach back to the original element.

Now onto extensions.

atom:extension rdfs:type atom:extensionElementProperty;

<a:entry>
<gd:when startTime="2005-06-06" endTime="2005-06-07"
        valueString="This weekend"/>
</a:entry>

[ :extension "<gd:when startTime="2005-06-06" endTime="2005-06-07"
        valueString="This weekend" xmlns:gd="http://schemas.google.com/g/2005"
/>"^^rdf:XMLLiteral .
]

Luckily, the only two things we need to pass to extension elements:
xml:lang and xml:base and those can easily be added (unless there's a
conflict, but that's another hill to climb and we're not there yet since
we can only climb on hill at a time).

Now in case someone brings up the I would like to query the name of the
extension (local and/or ns) etc, I would like to push that off to SPARQL
and a XPath extension against the XMLLiteral.

Additionally, we could support :simpleExtension.

<a:entry>
<s:when>Today</s:when>
</a:entry>

[
   s:when "Today" .
] .

s:when rdf:type atom:simpleExtensionElementProperty .
atom:simpleExtensionElementProperty rdfs:subClassOf rdf:Property .

Regards,