theorem BOOLE'69: :: BOOLE'69: X \/ (X /\ Y) = X proof thus X \/ (X /\ Y) c= X proof let x; assume x in X \/ (X /\ Y); then x in X or x in X /\ Y by XBOOLE_0:def 2; hence thesis by XBOOLE_0:def 3; end; let x; thus thesis by XBOOLE_0:def 2; end;これは以下の通りです。