theorem :: BOOLE'25: (for x holds not x in X iff (x in Y iff x in Z)) implies X = Y \+\ Z proof assume A1: not x in X iff (x in Y iff x in Z); now let x; x in X iff x in Y & not x in Z or x in Z & not x in Y by A1; then x in X iff x in Y \ Z or x in Z \ Y by Def4; then x in X iff x in (Y \ Z) \/ (Z \ Y) by Def2; hence x in X iff x in Y \+\ Z by Def6; end; hence thesis by TARSKI:2; end;