theorem :: BOOLE'59: X \/ Y = {} implies X = {} proof assume A1: X \/ Y = {}; not ex x st x in X proof given x such that A3:x in X; x in X \/ Y by XBOOLE_0:def 2,A3; hence contradiction by A1,XBOOLE_0:def 1; end; hence thesis by XBOOLE_0:def 1; end;これは以下の通りです。