theorem Th5: :: (2.1) (2.3 <=> 2.5)
  a <= b iff a < b & -b < -a
  proof
    thus a <= b implies a < b & -b < -a
    proof
      assume a <= b; then
      a =-> b = Top L by Def6;
      hence a < b & -b < -a by Th4;
    end;
    assume a < b & -b < -a; then
    a =-> b = Top L;
    hence thesis by Def6;
  end;
