theorem BOOLE'47: :: BOOLE'47: X c= Y implies Z \ Y c= Z \ X proof assume A1:X c= Y; let x; assume x in Z \ Y; then x in Z & not x in Y by XBOOLE_0:def 4; then x in Z & not x in X by A1,TARSKI:def 3; hence thesis by XBOOLE_0:def 4; end;これは以下の通りです。
証明