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

theorem
  X c= Y implies Y = X (\/) (Y (\) X)
proof
  assume
A1: X c= Y;
  thus Y = (Y (\) X) (\/) Y (/\) X by Th65
    .= X (\/) (Y (\) X) by A1,Th23;
end;
