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[protege-owl] How to make a restriction for "A HasPrerequisite (X and Y and Z)" expression

Thomas Russ tar at ISI.EDU
Tue May 27 14:35:53 PDT 2008


On May 27, 2008, at 10:50 AM, ozg wrote:

>
> I'm new and developing an university education ontology.
> I added every lecture as a class to the ontology. And I defined a  
> property
> "HasPrerequisite" meaning some lectures can have 1 or more  
> prerequisite(s).
> If someone wants to take a lecture he/she must have passed from all  
> of the
> prerequisite lectures. I defined HasPrerequisite property from  
> lecture to
> lecture. And by making restrictions I adapt the property for each  
> lecture
> class.
> My question is:
> If a lecture has only 1 prerequisite lecture, I define a hasValue
> restriction. Right?
> But if a lecture has many prerequisites I can't use hasValue with
> anding(intersecting) them. A person can take a lecture if the person  
> passed
> all of the prerequisite lectures. How can I define such a  
> restriction? Like
> "LectureA HasPrerequisite (LectureX and LectureY and LectureZ)"
>
> By making hasValue restrictions for all of the prerequisites one by  
> one.
> Or and'ing them and using maybe allValuesFrom restriction.

Well, this is a bit of a tricky modeling situation.

But first, let's be clear about how some of the different restrictions  
work.

hasValue takes an Individual as its argument and means that the  
restricted property must have that individual as one of its fillers.

someValueFrom takes a Class as its argument and means that the  
restricted property must have at least one filler from that class.

Generally, unless you want to create OWL-Full ontologies, you will not  
be using hasValue with classes.  You would generally be using  
someValuesFrom instead.

But some of that depends on what other choices you make in your  
modeling.  In other words, it partly depends on what your domain of  
discourse is, and that is where some of the choices get a bit tricky.

OPTION 1.  Make each lecture an individual.  In this case you will be  
treating each lecture as an individual rather than as a class of  
lectures.  It will also mean that you can't distinguish between a  
lecture given in 2006 and one given in 2008, since there will only be  
one individual for each lecture.

If you go this route, then you can use hasValue to set the values of  
the pre-requisites.   You would have a separate hasValue restriction  
for each pre-requisite.  You can have multiple such restrictions.    
Students can take these lectures, but you won't be able to distinguish  
among lectures taken in different years or semesters.   You also would  
not be able to tell if two students took the lecture at the same  
time.   In effect, you will have treated the lecture class as the  
concrete domain object that you are talking about.

But this is still problematic because pre-requisites are not  
restrictions on the lectures, but rather on the students who take the  
lectures.  See discussion below.

OPTION 2.  Make each lecture a class.  The individuals of these  
classes can be particular instances of the lectures happening, and  
thus have particular instructors, students, times, etc.  Pre- 
requisites could perhaps then be modeled using someValuesFrom type  
restrictions.

But this approach is problematic because the pre-requisites are NOT  
actually restrictions on the lectures themselves, but rather  
restrictions on the students who are taking the lectures.  So one  
would need to use a more complicated formulation to place the  
restriction on the student.  I think this may be the most accurate  
model, but the formulation makes the pre-requisites be an implicit  
model, which would make it difficult to answer the question "What are  
the pre-requisites of course X?".  That is because one would otherwise  
require a higher order logic to represent the mapping rule.

This relies on the presence of some additional properties:
     takesCourse  (Student -> Lecture)
     hasStudent   (Lecture -> Student), inverse of takesCourse

To say that Lecture1 has Lecture2 as a pre-requisite, one could then  
write:
     Lecture1 <=> hasStudent allValuesFrom (Student and takesCourse  
someValuesFrom Lecture2)

In other words, students that are taking (an instance of) Lecture1  
must belong to the class of students how have taken (an instance of)  
Lecture2.  Now this can be extended to a requirement of having passed  
the class by just changing the "takesCourse" property to  
"passesCourse" in the restriction.

This extends to multiple pre-requisites by just adding additional  
someValuesFrom clauses:

    LectureA <=> hasStudent allValuesFrom (Student
                                           and passesCourse  
someValuesFrom LectureX
                                           and passesCourse  
someValuesFrom LectureY
                                           and passesCourse  
someValuesFrom LectureZ)

This gives a good structural model, although as noted above, it would  
not be easy to query the ontology to find the restrictions from the  
someValues from clauses.


OPTION 3.  Make the pre-requisites annotations of the lecture classes  
rather than restrictions.  In this case you would have to build your  
own inference procedure to check for consistency, because annotations  
don't get enforced by the classification reasoner the way restrictions  
do.  Also, not all systems are happy having classes as values of  
annotation properties.  Since you have your own interpretation of the  
annotation, you can assign the appropriate semantics to the  
interpretation and enforcement of the pre-requisites property.

OPTION 4.  Use classes as fillers of the pre-requisites property.   
This puts you into OWL-Full and still doesn't solve the issue of the  
student being the one restricted.





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