theorem Th33:
  not p,q '||' p,r & p,r '||' p,s & q,r '||' q,s implies r=s
proof
  assume that
A1: ( not p,q '||' p,r)& p,r '||' p,s and
A2: q,r '||' q,s;
A3: r,s '||' r,q by A2,Th29;
  ( not r,p '||' r,q)& r,s '||' r,p by A1,Th29;
  hence thesis by A3,Def11;
end;
