theorem BOOLE'40: :: BOOLE'40: 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 & x in Z by XBOOLE_0:def 3; then x in Y & x in Z by A1,TARSKI:def 3; hence thesis by XBOOLE_0:def 3; end;