 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 Th23: :: (2.23)
  ((Top L) => b) = b
  proof
    (b "/\" (Top L)) < b by Def2; then
A1: b < ((Top L) => b) by Def4;
    -((Top L) => b) < ((Top L) "/\" -b) by Def10; then
A2: b <= ((Top L) => b) by A1,Th5;
A3: (Top L) "/\" ((Top L) => b) < b by Th17;
    (Top L) "/\" -b < -((Top L) => b) by Def9; then
    ((Top L) => b) <= b by Th5,A3;
    hence thesis by A2,Th3;
  end;
