theorem :: BOOLE'57: X = Y /\ Z iff X c= Y & X c= Z & for V st V c= Y & V c= Z holds V c= X proof thus X = Y /\ Z implies X c= Y & X c= Z & for V st V c= Y & V c= Z holds V c= X by BOOLE'37,BOOLE'39; assume that A1: X c= Y and A2: X c= Z and A3: V c= Y & V c= Z implies V c= X; thus X c= Y /\ Z by BOOLE'39,A1,A2; Y /\ Z c= Y & Y /\ Z c= Z implies Y /\ Z c= X by A3; hence Y /\ Z c= X by BOOLE'37; end;これは以下の通りです。
証明終了