theorem Th20:
  not s ==>. t, {}(E^omega, E^omega)
proof
  assume s ==>. t, {}(E^omega, E^omega);
  then consider v, w, s1, t1 such that
  s = v^s1^w and
  t = v^t1^w and
A1: s1 -->. t1, {}(E^omega, E^omega);
  [s1, t1] in {}(E^omega, E^omega) by A1;
  hence contradiction by PARTIT_2:def 1;
end;
