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When sending a TX, there's an optional gasPrice field. If I don't set it, what's it default to? Does it look at other TXs going on or is it a fixed value that only changes when web3 gets updated to a new version? Is there a way to get the default value so that I can, for example, add 1gwei to it? I'd like to ensure order of TXs in the same block by paying more for the first TX.