let X be Subset of CQC-WFF; :: thesis: X c= Cn X
let a be set ; :: according to TARSKI:def 3 :: thesis: ( not a in X or a in Cn X )
assume A1: a in X ; :: thesis: a in Cn X
then reconsider t = a as Element of CQC-WFF ;
for T being Subset of CQC-WFF st T is being_a_theory & X c= T holds
t in T by A1;
hence a in Cn X by Def2; :: thesis: verum