reserve i,j,e,u for object;
reserve I for set; 
reserve x,X,Y,Z,V for ManySortedSet of I;

theorem Th65:
  (X (\) Y) (\/) X (/\) Y = X
proof
  thus X (\) Y (\/) X (/\) Y = X (\) (Y (\) Y) by Th64
    .= X (\) EmptyMS I by Th52
    .= X by Th59;
end;
