 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

theorem  :: (2.9)
  a =-> b = Top L & b =-> a = Top L implies a = b
  proof
    assume a =-> b = Top L & b =-> a = Top L; then
    a "/\" b = a & b "/\" a = b by Def6;
    hence thesis;
  end;
