theorem
  X (\/) Y = X (\+\) Y (\+\) X (/\) Y
proof
  thus X (\/) Y = X (\+\) (Y (\) X) by Th92
    .= X (\+\) (Y (\+\) X (/\) Y) by Th94
    .= X (\+\) Y (\+\) X (/\) Y by Th90;
end;
