theorem Th67:
  X (\/) (Y (\) X) = X (\/) Y
proof
  thus X (\/) (Y (\) X) = X (\/) Y (/\) X (\/) (Y (\) X) by Th31
    .= X (\/) (Y (/\) X (\/) (Y (\) X)) by Th28
    .= X (\/) Y by Th65;
end;
