theorem Th87:
  X (\+\) Y = (X (\/) Y) (\) X (/\) Y
proof
  thus X (\+\) Y = (X (\) X (/\) Y) (\/) (Y (\) X) by Th70
    .= (X (\) X (/\) Y) (\/) (Y (\) X (/\) Y) by Th70
    .= (X (\/) Y) (\) X (/\) Y by Th72;
end;
