docs: @Override
  	public boolean checkMgtKeyInUse(String CorpNum, int ItemCode, String MgtKey)
  			throws PopbillException {
  		if (MgtKey == null || MgtKey.isEmpty())
  			throw new PopbillException(-99999999, "관리번호가 입력되지 않았습니다.");
  		
  		try{
  			StatementInfo info = httpget("/Statement/" + ItemCode + "/" + MgtKey, 
  					CorpNum, null, StatementInfo.class);
  			
  			return (info.getItemKey() == null || info.getItemKey().isEmpty()) == false;
  			
  			} catch(PopbillException PE){
  			if(PE.getCode() == -12000004)
  				return false;
  			throw PE;
  		}
  		
  	}