
theorem Jajo:
  for A,B,C being set holds
    A \+\ C = (A \+\ B) \+\ (B \+\ C)
  proof
    let A,B,C be set;
    (A \+\ B) \+\ B = A \+\ (B \+\ B) by XBOOLE_1:91
      .= A \+\ {} by XBOOLE_1:92
      .= A;
    hence thesis by XBOOLE_1:91;
  end;
