theorem BOOLE'46: :: BOOLE'46: X c= Y implies X \ Z c= Y \ Z proof assume A1:X c= Y; let x; assume x in X \ Z; then x in X & not x in Z by XBOOLE_0:def 4; then x in Y & not x in Z by A1,TARSKI:def 3; hence thesis by XBOOLE_0:def 4; end;