theorem BOOLE'35: :: BOOLE'35: X c= Y implies X \/ Y = Y proof assume A1: X c= Y; thus X \/ Y c= Y proof let x; assume x in X \/ Y; then x in X or x in Y by XBOOLE_0:def 2; hence thesis by A1,TARSKI:def 3; end; let x; thus thesis by XBOOLE_0:def 2; end;これは以下の通りです。