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

theorem
  X (\) Y misses Y (\) X
proof
  let i be object;
  assume i in I;
  then (X (\) Y).i = X.i \ Y.i & (Y (\) X).i = Y.i \ X.i by Def6;
  hence thesis by XBOOLE_1:82;
end;
