theorem Th35: :: (2.36)
  ((-(a => b)) => (a "/\" -b)) "/\" ((a "/\" -b) => -(a => b)) = Top L
  proof
A1: -(a => b) < (a "/\" -b) by Def10;
    (a "/\" -b) < -(a => b) by Def9;
    hence thesis by A1;
  end;
