
theorem
  for L being Boolean LATTICE, x being Element of L holds 'not'(x~) = 'not' x
proof
  let L be Boolean LATTICE, x be Element of L;
  for x being Element of L holds 'not' 'not' x = x by WAYBEL_1:87;
  then 'not' x is_a_complement_of x by WAYBEL_1:86;
  then ('not' x)~ is_a_complement_of x~ by Th35;
  hence thesis by YELLOW_5:11;
end;
