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 Th21:
  Cn(X) \/ Cn(Y) c= Cn(X \/ Y)
proof
A1: Cn(Y) c= Cn(X \/ Y) by CQC_THE1:18,XBOOLE_1:7;
  Cn(X) c= Cn(X \/ Y) by CQC_THE1:18,XBOOLE_1:7;
  hence thesis by A1,XBOOLE_1:8;
end;
