reserve Al for QC-alphabet;
reserve i,j,n,k,l for Nat;
reserve a for set;
reserve T,S,X,Y for Subset of CQC-WFF(Al);
reserve p,q,r,t,F,H,G for Element of CQC-WFF(Al);
reserve s for QC-formula of Al;
reserve x,y for bound_QC-variable of Al;

theorem Th13:
  X c= Cn(X)
proof
  let a be object;
  assume
A1: a in X;
  then reconsider t=a as Element of CQC-WFF(Al);
 for T st T is being_a_theory & X c= T holds t in T by A1;
  hence thesis by Def2;
end;
