The authors define the abstract syntax of USL using a metamodel together with OCL wellformedness rules and then provide a graphical concrete syntax
for the usability goal.
9) we have chosen a concrete syntax
for process modelling with special visual effects for model elements.
and exchange format are described in section 6.
The RDF data model provides an abstract, conceptual framework; a concrete syntax
is also required and XML is used for this purpose.
Ideally, SGML editing systems support at least the Reference Concrete Syntax
: "SGML, being an abstract language, refers to characters by their roles rather than by their graphical representation.
First, regarding to USL specification, we make the abstract syntax precise with the OCL wellformedness rules  and define a graphical concrete syntax
. Second, we develop an additional case study in order to illustrate how to apply USL in practice.
Designing a framework is like designing a programming language, except that the only concrete syntax
is the one used to implement the framework.
This meta-language must precisely specify semantics, but must not otherwise constrain the concrete syntax
used by any particular programming language.
Since the literal basic symbols are enclosed in apostrophes in the concrete syntax
of Figure 1, it is a simple matter for an appropriate tool to extract them.