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

theorem Th108:
  Y misses X (\) Y
proof
  now
    let i be object;
    assume i in I;
    then (X (\) Y).i = X.i \ Y.i by Def6;
    hence Y.i misses (X (\) Y).i by XBOOLE_1:79;
  end;
  hence thesis;
end;
