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

theorem Th70:
  X (\) X (/\) Y = X (\) Y
proof
  thus X (\) X (/\) Y = (X (\) X) (\/) (X (\) Y) by Th69
    .= EmptyMS I (\/) (X (\) Y) by Th52
    .= X (\) Y by Th22,Th43;
end;
