1% { a, b, c, not d, not e }, { a, c, d, not b, not e }
    2% test of factorization
    3a :- b, c.
    4a :- d, c.
    5a :- c, b, e.
    6
    7c :- not e.
    8e :- not c.
    9
   10b :- not d.
   11d :- not b.
   12
   13% #compute 2 {a}.
   14:- not a.