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

theorem
  X (\+\) EmptyMS I = X
proof
  thus X (\+\) EmptyMS I = X (\/) (EmptyMS I (\) X) by Th59
    .= X (\/) EmptyMS I by Th60
    .= X by Th22,Th43;
end;
