1. An Interactive Mathematical Proof System, http://imps.mcmaster.ca/
2. The Coq Proof Assistant, http://coq.inria.fr/
3. The MIZAR Project, http://www.mizar.org/
4. The OMEGA System, http://www.ags.uni-sb.de/~omega/
5. Bledsoe, W.W., Feng, G.: SET-VAR. J. Automated Reasoning 11(3), 293–314 (1993)