function approve(address _to, uint256 _tokenId)
        senderVerify()
        public
    {
        require (register[_to] != bytes32(0), "Not a registered user");
        require (msg.sender == cardList[_tokenId].playerAddress, "The card does not belong to you");
        require (cardList.length > _tokenId, "tokenId error");
        require (cardIndexToApproved[_tokenId] == address(0), "Approved");
        
        cardIndexToApproved[_tokenId] = _to;
        
        emit Approval(msg.sender, _to, _tokenId);
    }