theorem :: BOOLE'41: X c= Y & Z c= V implies X /\ Z c= Y /\ V proof assume A1: X c= Y & Z c= V; 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 V by A1,TARSKI:def 3; hence thesis by XBOOLE_0:def 3; end;これは以下の通りです。