cnf dnf

https://en.wikipedia.org/wiki/Boolean_satisfiability_problem#Horn-satisfiability

https://www7.in.tum.de/um/courses/logic/SS11/folien/horn-english.pdf

https://math.stackexchange.com/questions/3460770/converting-to-dnf-from-cnf

https://cs.stackexchange.com/questions/41070/dnf-to-cnf-conversion-easy-or-hard

https://www.mpi-inf.mpg.de/fileadmin/inf/rg1/script4ws1617.pdf

https://stackoverflow.com/questions/11863421/boolean-function-what-is-the-purpose-of-dnf-and-cnf

https://www.cpp.edu/~jrfisher/www/prolog_tutorial/logic_topics/normal_forms/normal_form.html

www.opentextbooks.org.hk/ditatopic/9544