theorem :: BOOLE'34: X c= Y & Z c= V implies X \/ Z c= Y \/ V proof assume A1: X c= Y; assume A2: Z c= V; 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 V by A1,A2,TARSKI:def 3; hence thesis by XBOOLE_0:def 2; end;これは以下の通りです。