theorem :: BOOLE'90: X \ Y = Y \ X implies X = Y proof assume A1: X \ Y = Y \ X; now let x; (x in X & not x in Y) iff x in Y \ X by XBOOLE_0:def 4,A1; hence x in X iff x in Y by XBOOLE_0:def 4; end; hence thesis by TARSKI:2; end;
証明