theorem :: BOOLE'72: (X /\ Y) \/ (Y /\ Z) \/ (Z /\ X) = (X \/ Y) /\ (Y \/ Z) /\ (Z \/ X) proof thus X /\ Y \/ Y /\ Z \/ Z /\ X = (X /\ Y \/ Y /\ Z \/ Z) /\ (X /\ Y \/ Y /\ Z \/ X) by BOOLE'71 .= (X /\ Y \/ (Y /\ Z \/ Z)) /\ (X /\ Y \/ Y /\ Z \/ X) by BOOLE'64 .= (X /\ Y \/ Z) /\ (X /\ Y \/ Y /\ Z \/ X) by BOOLE'69 .= (X /\ Y \/ Z) /\ (X /\ Y \/ X \/ Y /\ Z) by BOOLE'64 .= (X /\ Y \/ Z) /\ (X \/ Y /\ Z) by BOOLE'69 .= (X \/ Z) /\ (Y \/ Z) /\ (X \/ Y /\ Z) by BOOLE'71 .= (X \/ Z) /\ (Y \/ Z) /\ ((X \/ Y) /\ (X \/ Z)) by BOOLE'71 .= (X \/ Y) /\ ((Y \/ Z) /\ (X \/ Z) /\ (X \/ Z)) by BOOLE'67 .= (X \/ Y) /\ ((Y \/ Z) /\ ((X \/ Z) /\ (X \/ Z))) by BOOLE'67 .= (X \/ Y) /\ (Y \/ Z) /\ (Z \/ X) by BOOLE'67; end;これは以下の通りです。