function update(address[] ofAssets, uint[] newPrices)
        pre_cond(isOwner())
        pre_cond(ofAssets.length == newPrices.length)
    {
        updateId += 1;
        for (uint i = 0; i < ofAssets.length; ++i) {
            require(information[ofAssets[i]].timestamp != now); 
            require(information[ofAssets[i]].exists);
            information[ofAssets[i]].timestamp = now;
            information[ofAssets[i]].price = newPrices[i];
        }
        PriceUpdated(now);
    }