function _deleteOffer(uint _unicornId) internal {
        if (offers[_unicornId].exists) {
            offers[market[--marketSize]].marketIndex = offers[_unicornId].marketIndex;
            market[offers[_unicornId].marketIndex] = market[marketSize];
            delete market[marketSize];
            delete offers[_unicornId];
            emit OfferDelete(_unicornId);
        }
    }