theorem Th75:
  (X (\/) Y) (\) Y = X (\) Y
proof
  thus (X (\/) Y) (\) Y = X (\) Y (\/) (Y (\) Y) by Th72
    .= X (\) Y (\/) EmptyMS I by Th52
    .= X (\) Y by Th22,Th43;
end;
