    function _removeAsset(address _asset)
        internal
        onlyWhenAssetExisted(_asset)
    {
        uint8 assetIndex = assetFeederMap[_asset].index;
        delete assets[assetIndex];
        delete assetPriceMap[_asset];
        delete assetFeederMap[_asset];
        emit AssetRemoved(_asset);
    }
    /// @dev Original raw value to aggregate with
    // the NFT contract address -> FeederRegistrar which contains price from each feeder
    mapping(address => FeederRegistrar) public assetFeederMap;
struct FeederRegistrar {
    // if asset registered or not
    bool registered;
    // index in asset list
    uint8 index;
    // if asset paused,reject the price
    bool paused;
    // feeder -> PriceInformation
    mapping(address => PriceInformation) feederPrice;
}
    function _addAsset(address _asset)
        internal
        onlyWhenAssetNotExisted(_asset)
    {
        assetFeederMap[_asset].registered = true;
        assets.push(_asset);
        assetFeederMap[_asset].index = uint8(assets.length - 1);
        emit AssetAdded(_asset);
    }
    function _combine(address _asset, uint256 _twap)
        internal
        view
        returns (bool, uint256)
    {
        FeederRegistrar storage feederRegistrar = assetFeederMap[_asset];
        uint256 currentBlock = block.number;
        //first time just use the feeding value
        if (assetPriceMap[_asset].twap == 0) {
            return (true, _twap);
        }
        //use memory here so allocate with maximum length
        uint256 feederSize = feeders.length;
        uint256[] memory validPriceList = new uint256[](feederSize);
        uint256 validNum = 0;
        //aggeregate with price from all feeders
        for (uint256 i = 0; i < feederSize; i++) {
            PriceInformation memory priceInfo = feederRegistrar.feederPrice[
                feeders[i]
            ];
            if (priceInfo.updatedAt > 0) {
                uint256 diffBlock = currentBlock - priceInfo.updatedAt;
                if (diffBlock <= config.expirationPeriod) {
                    validPriceList[validNum] = priceInfo.twap;
                    validNum++;
                }
            }
        }
        if (validNum < MIN_ORACLES_NUM) {
            return (false, assetPriceMap[_asset].twap);
        }
        _quickSort(validPriceList, 0, int256(validNum - 1));
        return (true, validPriceList[validNum / 2]);
    }
    function _removeAsset(address _asset)
        internal
        onlyWhenAssetExisted(_asset)
    {
+       FeederRegistrar storage feederRegistrar = assetFeederMap[_asset];
+       uint256 feederSize = feeders.length;
+       for (uint256 i = 0; i < feederSize; i++) {
+           delete feederRegistrar.feederPrice[feeders[i]];
+       }
        uint8 assetIndex = feederRegistrar.index;
        delete assets[assetIndex];
        delete assetPriceMap[_asset];
        delete assetFeederMap[_asset];
        emit AssetRemoved(_asset);
    }
