reserve L for Boolean non empty RelStr;
reserve a,b,c,d for Element of L;

theorem
  'not' (a\b) = 'not' a"\/"b
proof
  thus 'not' (a\b) = 'not' a"\/"'not' 'not' b by Th36
    .= 'not' a"\/"b by WAYBEL_1:87;
end;
