reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem
  X c= Y implies Y = X \/ (Y \ X)
proof
  assume
A1: X c= Y;
  now
    let x be object;
    x in Y iff x in X or x in (Y \ X) by A1,XBOOLE_0:def 5;
    hence x in Y iff x in X \/ (Y \ X) by XBOOLE_0:def 3;
  end;
  hence thesis by TARSKI:2;
end;
