1. https://github.com/HyVar/hyvar-rec. https://github.com/HyVar/hyvar-rec.
2. http://deltaecore.org. http://deltaecore.org.
3. http://wwwiti.cs.uni-magdeburg.de/iti_db/research/\featureide/. http://wwwiti.cs.uni-magdeburg.de/iti_db/research/\featureide/.
4. https://www.pure-systems.com. https://www.pure-systems.com.
5. http://www.biglever.com/. http://www.biglever.com/.