theorem Th7:
  ex b9 st a9<>b9
proof
  consider p9,q9,r9 such that
A1: p9<>q9 and
  q9<>r9 and
  r9<>p9 by Th6;
  a9<>p9 or a9<>q9 by A1;
  hence thesis;
end;
