reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;
reserve T, X, Y for Subset of MC-wff;
reserve p, q, r for Element of MC-wff;

theorem
  CnCPC(CnCPC(X)) = CnCPC(X)
proof
A1: CnCPC(X) c= CnCPC(CnCPC(X)) by Th72;
  CnCPC(CnCPC(X)) c= CnCPC(X) by Lm21;
  hence thesis by A1,XBOOLE_0:def 10;
end;
