1. Object Technology Series;G. Booch,1999
2. UML 2.0 Infrastructure Specification – OMG Adopted Specification. OMG Document ptc/03-09-15 (September 2003)
3. UML 2.0 Superstructure Specification – OMG Adopted Specification. OMG Document ptc/03-08-02 (August 2003)
4. UML 2.0 OCL Specification – OMG Final Adopted Specification. OMG Document ptc/03-10-14 (October 2003)
5. Overview on current OCL tools,
http://www.klasse.nl/ocl/ocl-services.html