1. Alf homepage.
http://homepages.inf.ed.ac.uk/wadler/realworld/alf.html
. Accessed 25 Oct 2014
2. Archive of formal proofs.
http://afp.sourceforge.net
. Accessed 18 May 2016
3. Coq homepage.
http://coq.inria.fr/
. Accessed 25 Oct 2014
4. HOL4 homepage.
http://hol.sourceforge.net/
. Accessed 25 Oct 2014
5. Isabelle homepage.
http://isabelle.in.tum.de/
. Accessed 20 Oct 2014