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

theorem
  X (/\) Y = EmptyMS I iff X (\) Y = X
proof
  hereby
    assume
A1: X (/\) Y = EmptyMS I;
    thus X (\) Y = X (\) X (/\) Y by Th70
      .= X by A1,Th59;
  end;
  thus thesis by Th63;
end;
