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A common technique for specification of languages is to first define the syntax of the language and then to describe its static
and dynamic semantics. The syntax defines what constructs exist in the language and how the constructs are built up in terms
of other constructs. Sometimes, especially if the language has a graphic syntax, it is important to define the syntax in a
notation independent way (i.e., to define the abstract syntax of the language). The concrete syntax is then defined by mapping
the notation onto the abstract syntax.
The static semantics of a language define how an instance of a construct should be connected to other instances to be meaningful,
and the dynamic semantics define the meaning of a well formed construct. The meaning of a description written in the language
is defined only if the description is well formed (i.e., if it fulfills the rules defined in the static semantics).
1. By definition semantic variation points are an exception to this.
The specification uses a combination of languages - a subset of UML, an object constraint language, and precise natural language
to describe the abstract syntax and semantics of the full UML. The description is self-contained; no other sources of information
are needed to read the document2. Although this is a metacircular description3, understanding this document is practical since
only a small subset of UML constructs are needed to describe its semantics.
In constructing the UML metamodel different techniques have been used to specify language constructs, using some of the capabilities
of UML. The main language constructs are reified into metaclasses in the metamodel. Other constructs, in essence being variants
of other ones, are defined as stereotypes of metaclasses in the metamodel. This mechanism allows the semantics of the variant
construct to be significantly different from the base metaclass. Another more “lightweight? way of defining variants is to
use metaattributes. As an example, the aggregation construct is specified by an attribute of the metaclass AssociationEnd,
which is used to indicate if an association is an ordinary aggregate, a composite aggregate, or a common association.