reserve T, X, Y for Subset of HP-WFF;
reserve p, q, r, s for Element of HP-WFF;

theorem
  CnPos(CnPos(X)) = CnPos(X)
proof
A1: CnPos(X) c= CnPos(CnPos(X)) by Th9;
  CnPos(CnPos(X)) c= CnPos(X) by Lm2;
  hence thesis by A1,XBOOLE_0:def 10;
end;
