The metamodels fully specify the domain modeling language, or more precisely, its
concrete syntax. They do not, at least not entirely, specify the static semantics of
the language. By static semantics we mean the set of rules that specify
the well-formedness of domain models. UML class diagrams do allow the specification
of some basic rules, for example, the multiplicity of associations. For more complex
semantic specifications, however, UML includes the Object Constraint Language (OCL),
a textual predicate logic language. MIC adopts OCL as well; metamodels consist of UML
class diagrams and OCL constraints.
Consider the example depicted in the figure on the MetaGME homepage. Suppose the finite state
machines in the target domain must not allow state transitions from a state to itself.
A UML class diagram alone cannot specify such a rule. Rather, the following OCL
expression must be attached to States:
self.transTo->forAll(s | s <> self)
where self and forAll are OCL keywords, while transTo is a role name of the Transition
association. These constraints are relatively straightforward to check programmatically.
The expressions need to be evaluated in the context of every applicable model object.
Notice that this problem is quite different from the well-known constraint satisfiability
problem. Its time-complexity is polynomial in the number of model objects.
