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

theorem Th68:
  X (\) (X (\) Y) = X (/\) Y
proof
  thus X (\) (X (\) Y) = (X (\) X) (\/) X (/\) Y by Th64
    .= EmptyMS I (\/) X (/\) Y by Th52
    .= X (/\) Y by Th22,Th43;
end;
