reserve A for QC-alphabet;
reserve p, q, r, s, p1, q1 for Element of CQC-WFF(A),
  X, Y, Z, X1, X2 for Subset of CQC-WFF(A),
  h for QC-formula of A,
  x, y for bound_QC-variable of A,
  n for Element of NAT;

theorem
  X |-| Cn(X)
proof
  Cn(X) = Cn(Cn(X)) by CQC_THE1:19;
  hence thesis by Th20;
end;
