- https://learnxinyminutes.com/docs/prolog/
- http://lpn.swi-prolog.org/lpnpage.php?pageid=online
- https://github.com/dvorka/parser-combinators/blob/768728eb7097b1919a28536d5cba548ca28338b7/src/prolog/demo/selfOn/bnf/prolog.bnf
- https://www.cs.cmu.edu/Groups/AI/util/lang/prolog/doc/intro/prolog.doc (1982)
- http://www.cs.cornell.edu/courses/cs3110/2011fa/supplemental/lec26-type-inference/type-inference.htm