WSML – a Language for WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 85

external to WSML; WSML allows reference to any choreography or orches-
tration identified by an IRI.
Goals
Goals are symmetric to Web services in the sense that goals describe the de-
sired functionality and Web services describe the offered functionality. There-
fore, a goal description consists of the same modeling elements as does a
Web service description, namely nonfunctional properties, a capability, and a
number of interfaces.
Mediators
Mediators connect different goals, Web services, and ontologies, enabling inter-
operation by reconciling differences in representation formats, encoding styles,
business protocols, etc. Connections between mediators and other WSML el-
ements can be established in two different ways:
1. Each WSML element allows the specification of a number of mediators to
be used through the keyword usesMediator .
2. Each mediator has, depending on the type of mediator, zero, one, or more
sources and zero or one target. The source and target are optional, in
order to allow generic mediators.
A mediator achieves its mediation functionality either through a Web ser-
vice, which provides the mediation service, or a goal, which can be used to
dynamically discover the appropriate (mediation) Web service.
7.2.3 Logical-Expression Syntax
We shall first explain the general logical-expression syntax, which encompasses
all WSML variants, and then describe the restrictions on this general syntax
for each of the variants. The general-logical expression syntax for WSML has
a first-order logic style, in the sense that it has constants, function symbols,
variables, predicates and the usual logical connectives. Furthermore, WSML
has extensions based on F-Logic [72] in order to model concepts, attributes, at-
tribute definitions, and subconcept and concept membership relationships. Fi-
nally, WSML has a number of connectives to facilitate the logic programming
based variants, namely default negation (negation-as-failure), LP-implication
(which differs from classical implication), and database-style integrity con-
straints.
Variables in WSML start with a question mark, followed by an arbitrary
number of alphanumeric characters, for example ?x , ?name ,or ?123 .Freevari-
ables in WSML (i.e. variables which are not explicitly quantified), are implic-
itly universally quantified outside of the formula (i.e. the logical expression