theorem BOOLE'33: :: BOOLE'33: 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 or x in Z by XBOOLE_0:def 2; then x in Y or x in Z by A1,TARSKI:def 3; hence thesis by XBOOLE_0:def 2; end;これは以下の通りです。
証明終了