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

theorem
  (a"\/"b)\(a"/\"b) = (a\b)"\/"(b\a)
proof
  thus (a"\/"b)\(a"/\"b) = (a"\/"b)"/\"('not' a"\/"'not' b) by Th36
    .= ((a"\/"b)"/\"'not' a)"\/"((a"\/"b)"/\" 'not' b) by WAYBEL_1:def 3
    .= ((a"/\"'not' a)"\/"(b"/\"'not' a))"\/"((a"\/"b)"/\" 'not' b) by
WAYBEL_1:def 3
    .= (Bottom L"\/"(b"/\"'not' a))"\/"((a"\/"b)"/\"'not' b) by Th34
    .= (b\a)"\/"((a"\/"b)"/\"'not' b) by WAYBEL_1:3
    .= (b\a)"\/"((a"/\"'not' b)"\/"(b"/\" 'not' b)) by WAYBEL_1:def 3
    .= (b\a)"\/"((a"/\"'not' b)"\/"Bottom L) by Th34
    .= (a\b)"\/"(b\a) by WAYBEL_1:3;
end;
