https://www.cs.cmu.edu/afs/cs/project/jair/pub/volume4/kaelbling96a-html/node25.html
https://www.cs.cmu.edu/afs/cs/project/jair/pub/volume4/kaelbling96a-html/node24.html
https://www.cs.cmu.edu/afs/cs/project/jair/pub/volume4/kaelbling96a-html/node25.html
https://www.cs.cmu.edu/afs/cs/project/jair/pub/volume4/kaelbling96a-html/node24.html