Fine-Grained Diagnostics of Ontologies with Assurance - Efficient Decision Support Systems: Practice and Challenges from Current to Future - page 82

fed into the reasoning engines. The context KB provides the service that other components
can query, add, delete or modify context knowledge stored in the context database (cf. Wang
et al. (2004)).
However, the problem of the current rules and reasoning systems is that they do not offer
high performance according the highly declarative way of the encoding of the problem and
the execution time to find the "best" solution.
As an example the used semantic web rule language (SWRL) has no negation or disjunction
and the arithmetic predicates which it offers are weak.

Obstacles in writing logical programs
Semantic web technology is widely used for reasoning and expert systems. Especially the use
of SWRL to define relationships between classes and individuals in the ontology may cause
major problems due to the long execution time of semantic web technology for querying the
ontology (e.g. via SWRL and SPARQL). This is indeed a problem when creating practical
expert systems ought to perform in real-time. Some existing paradigms as well suffer from
negation as failure, which has been discussed by Matheus et al. (2005). Moreover, they not
necessarily allow n -ary predicates within a rule.
Going away from SWRL, we still require constraints, negation as failure, and expressive
representation that are decidable and permit reasoning (efficiently). In particular, some logical
programming languages do not offer arithmetic operations "built-in", and numeric constraints
can affect decidability. Since reasoning (like done in the Prolog-language for instance), is often
a recursive procedure, its complexity can become exponential and thus infeasible for even a
medium-size ontology.
Now, to avoid the weaknesses of the existing paradigms answer set programming (ASP) offers
flexible and high performance reasoning. ASP is a declarative approach for modeling and
solves search problems by representing them as logic programs.

2.1 Answer set programming
The importance of ASP lies in the fact that it provides meaning to logic programs with default
negation "not". Many interesting applications exist in planning, reasoning about action,
configuration, diagnosis, space shuttle control, spatial, temporal and probabilistic reasoning,
constraint programming, etc.
The Technical University of Vienna (TU-Wien) hosts a the research group "knowledge
based systems", whose members are running a project on "Answer Set Programming for
the Semantic Web". The goal of this project is research towards methods for providing
advanced reasoning services in the context of the Semantic Web, using declarative knowledge
representation and reasoning techniques (see Eiter et al. (2005)).
A logic program in the language of AnsProlog (also known as A-Prolog ) is a set of rules of the
form:

← a 0 a 1 ,..., a m , not a m + 1 , . . . , not a n (1)

where 0 n , each a i is an atom of some propositional language and not represents

negation-as-failure . A negation-as-failure literal (or naf-literal ) has the form not a , where a is an
atom. Given a rule of this form, the left and right hand sides are called the head and body ,
respectively. A rule may have either an empty head or an empty body, but not both. Rules
with an empty head are called constraints, while those with an empty body are known as facts .
A definite rule is a rule which does not contain naf-literals, and a definite program is solely
composed of definite rules (cf. Baral et al. (2010)).

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