Thanks for sharing this. Read it through and sort of follow (non-tech person here :).. except I don’t understand why transactions are tracked in the context of their ancestors? Probably obvious, but could you, or someone, pls explain how a future fee rate is/can be estimated by a past fee rate? Thanks! 

 Transaction B may depend on some prior transaction A which is not yet confirmed. Imagine a chain of unconfirmed transactions. Mempool has to track them all.