theorem
  Bottom NormForm A = {}
proof
  {} in Normal_forms_on A by Lm4;
  then reconsider Z = {} as Element of NormForm A by Def12;
  now
    let u be Element of NormForm A;
    reconsider z = Z, u9 = u as Element of Normal_forms_on A by Def12;
    thus Z "\/" u = mi (z \/ u9) by Def12
      .= u by Th42;
  end;
  hence thesis by LATTICE2:14;
end;
