theorem :: SYSREL'2: (X \/ Y) \/ Z = (X \/ Z) \/ (Y \/ Z) proof (X \/ Y) \/ Z = X \/ ((Z \/ Z) \/ Y) by BOOLE'64 .= X \/ (Z \/ (Z \/ Y)) by BOOLE'64 .= (X \/ Z) \/ (Y \/ Z) by BOOLE'64; hence thesis; end;