1% { q, r, not p, not s }
    2% Two even loops allow selection, but the OLON forces r to succeed.
    3
    4p :- not q.
    5q :- not p.     % (p ^ not q) V (q ^ not p)
    6r :- not s, q.
    7s :- not r.     % (r ^ not s) V (s ^ not r)
    8
    9% #compute 1 { r }.
   10:- not r.