The Semantic Web - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 33

person[ child = >> person].
parent :: person.
X:parent : − X[child −>> Y], X:person.
Listing 3.2. Example F-Logic ontology
Because SWRL combines the full expressive power of function-free Horn
logic with an expressive description logic language, the key inferences tasks
(e.g. satisfiability and entailment) are in general undecidable for SWRL.
3.4.2 F-Logic
F-Logic [72], and, more specifically, the Horn subset of F-Logic extended with
negation, has been proposed as an ontology and rule language for the Semantic
Web [69]. Rules in F-Logic are similar to Horn rules, with the distinction that
besides atomic formulas, F-Logic rules also allow molecules in place of atomic
formulas. Note that although the syntax of F-Logic seems higher-order, the
language remains semantically in the first-order framework.
There are various kinds of molecules. An is-a assertion C:D states that
C is an instance of the class D ; a subclass assertion C::D states that C is a
subclass of D . Data molecules of the form C[D ->> E] have the meaning that
the attribute D of the individual C has the value E . Signature molecules of
the form C[D =>> E] indicate that the class C has an attribute D and that all
values associated with this attribute are of type E .
To simplify matters, we shall focus only on a subset of F-Logic. We do
not consider parameterized methods or functional (single-valued) methods,
but instead consider only noninheritable methods. We also do not consider
compound molecules.
An important concept in F-Logic is object identity [68]. Each object (e.g. a
class, instance, or method) has a unique object identifier, where an object
identifier is in fact a term. In F-Logic, classes and methods are interpreted
intentionally , which means that class identifiers and method identifiers are
interpreted by themselves and not directly as sets or as binary relations, as is
the case with concepts and roles in description logics. Classes and methods are
first interpreted as objects in the domain, and these objects are then related
to sets of objects and sets of binary tuples, respectively.
The simple F-Logic ontology shown in Listing 3.2 models the concept
person, which has an attribute child with type person. The concept parent is a
subconcept of person, and the rule states that every person with a child is a
parent.
Note that in F-Logic there is no distinction between classes and instances.
An object identifier can denote a class, an instance, or an attribute, but there
is no separation in the signature Σ of the identifiers denoting any of these. The
advantage of such an overloaded concept of an object is that objects denote