1. Knowledge Representation Part VI
Some more details when it comes to RDF, etc.
Jan Pettersen Nytun, UiA

2. Merging Graphs
[3]: When is a node in one graph the same node as a node in another graph?
In RDF, this is solved through the use of Uniform Resource Identifiers (URIs).

3. URIs Identify Web Resources
Web addresses are the most common URIs, i.e., uniform Resource Locators (URLs).
RDF resources are usually named by URI references (URIrefs). URIref = URI + optional fragment identifier at the end. E.g., were = URL tom = fragment identifier = namespace

4. Graph Merging The application is quite simple: A node from one graph is merged with a node from another graph—exactly, if they have the same URI.
[ref. emanueledellavalle.org]:
Propranolol (INN) is a sympatholytic nonselective beta blocker. It is used to treat high blood pressure, a number of heart dysrhythmias, thyrotoxicosis, and essential tremors.[1] It is used to prevent migraine headaches, and to prevent further heart problems in those with angina or previous heart attacks.[1] It comes in both oral and intravenous forms.
An anxiolytic (also antipanic or antianxiety agent)[1] is a medication or other intervention that inhibits anxiety.

5. Standard Namespace For RDF Predefined Terms
rdf: Indicates identifiers used in RDF. The set of RDF identifiers is small. The global URI for the rdf namespace is: If you open (e.g., in Internet Explorer) you may see the list of RDF identifiers.

6. RDF Identifier rdf:type
[Ref.

7. rdf:type in http://www.w3.org/1999/02/22-rdf-syntax-ns
rdf:type is mostly being described by RDFS contructs, e.g., rdfs:domain.

8. Reification
In general: Taking an abstract concept and making it concrete by having a proxy representing the abstract concept.
Reification (computer science) from Wikipedia: …process …abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. …By means of reification, something that was previously implicit, unexpressed,… is explicitly formulated and made available to conceptual (logical or computational) manipulation.

9. Reification Example 1
Many languages (e.g., Lisp) provide an eval that effectively reifies the language interpreter.

10. Reification Example 2
[ref Wikipedia]:
Reification is widely used in conceptual modeling.
Reifying a relationship means viewing it as an entity.
The purpose of reifying a relationship is to make it explicit, when additional information needs to be added to it.

11. Reification In RDF - 1
Making an assertion (statement) about an assertion.
Example [4]: «Wikipedia says Shakespeare wrote Hamlet»
Which express information about the statement: “Shakespeare wrote Hamlet”. The following triple does not cover this: :Shakespeare :wrote :Hamlet.
How to do this? Higher-order relationships may be used to solve this.

12. HIGHER-ORDER RELATIONSHIPS
rdf:statement: a triple with properties: rdf:subject, rdf:predicate, rdf:object rdf:subject, rdf:predicate, rdf:object : relate elements of statement tuple to a resource of type statement.

13. Reification In RDF - 2
“Wikipedia says Shakespeare wrote Hamlet.” Using the RDF standard, we can refer to a triple as follows: q:n1 rdf:subject lit:Shakespeare; rdf:predicate lit:wrote; rdf:object lit:Hamlet. web:Wikipedia m:says q:n1.
What is missing? Statement

14. Reification In RDF - 3
[4]: This kind of metadata about statements often takes the form of provenance (information about the source of a statement, as in this example), likelihood (expressed in some quantitative form like probability, such as It is 90 percent probable that Shakespeare wrote Hamlet), context, or time frame… In such cases, it is useful to explicitly make a statement about a statement. This process is called explicit reification.
provenance pronunciation: pråvenens

15. Another Example
[ref.
Explicit reification uses three properties,rdf:subject, rdf:predicate, and rdf:object, to define the three elements of a triple, and then assigns a URI to the triple. For example,
news:12345 rdf:subject bb:WhiteyFord;
rdf:predicat ebb:playsPosition;
rdf:object bb:Pitcher.
news:NewYorkTimes news:reports news:12345.
Reification is an important concept, since assertions often need to be made as to the source, date, confidence, etc. of assertions in the domain of discourse.

16. Ordered information in RDF
[4]: RDF provides a facility for ordering elements in a list format. E.g., In Turtle:
lit:Shakespeare b:hasChild (b:Susanna b:Judith b:Hamnet).
This translates into the following triples, where _:a, _:b, and _:c are bnodes:
lit:Shakespeare b:hasChild _:a.
_:a rdf:first b:Susanna.
_:a rdf:rest _:b.
_:b rdf:first b:Judith.
_:b rdf:rest _:c.
_:c rdf:rest rdf:nil.
_:c rdf:first b:Hamnet.

17. References
[1] David Poole and Alan Mackworth, Artificial Intelligence: Foundations of Computational Agents, Cambridge University Press, 2010, [2] [3] RDF 1.1 Primer, W3C Working Group Note, 24 June 2014 [4] Semantic Web for the Working Ontologist, Second Edition: Effective Modeling in RDFS and OWL, May 20, 2011, by Dean Allemang, James Hendler